Fix SS03 issues in docstrings
Fix the docstrings where summary does not end with a period.

Current errors found:
```
$ ./scripts/validate_docstrings.py --errors=SS03
None:None:SS03:pandas.tseries.offsets.DateOffset.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BusinessDay.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BusinessHour.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.CustomBusinessDay.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.CustomBusinessHour.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.MonthOffset.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.MonthEnd.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.MonthBegin.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BusinessMonthEnd.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BusinessMonthBegin.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.CustomBusinessMonthEnd.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.CustomBusinessMonthBegin.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.SemiMonthOffset.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.SemiMonthEnd.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.SemiMonthBegin.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.Week.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.WeekOfMonth.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.LastWeekOfMonth.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.QuarterOffset.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BQuarterEnd.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BQuarterBegin.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.QuarterEnd.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.QuarterBegin.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.YearOffset.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BYearEnd.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BYearBegin.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.YearEnd.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.YearBegin.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.FY5253.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.FY5253Quarter.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.Easter.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.Tick.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.Day.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.Hour.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.Minute.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.Second.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.Milli.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.Micro.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.Nano.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BDay.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BMonthEnd.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.BMonthBegin.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.CBMonthEnd.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.CBMonthBegin.normalize:Summary does not end with a period
None:None:SS03:pandas.tseries.offsets.CDay.normalize:Summary does not end with a period
None:None:SS03:pandas.Timestamp.isoweekday:Summary does not end with a period
None:None:SS03:pandas.Timestamp.weekday:Summary does not end with a period
None:None:SS03:pandas.DatetimeIndex.freqstr:Summary does not end with a period
None:None:SS03:pandas.PeriodIndex.freqstr:Summary does not end with a period
pandas/pandas/core/window/indexers.py:34:SS03:pandas.api.indexers.BaseIndexer:Summary does not end with a period
None:None:SS03:pandas.io.formats.style.Styler.loader:Summary does not end with a period
```