DOC: Fixed PR08 docstring errors in pandas.tseries
This relates to: [27977](https://github.com/pandas-dev/pandas/issues/27977). I have fixed the doc PR08 formatting issues for:

```
pandas.tseries.offsets.CustomBusinessDay: Parameter "weekmask" description should start with a capital letter
pandas.tseries.offsets.CustomBusinessDay: Parameter "holidays" description should start with a capital letter
pandas.tseries.offsets.WeekOfMonth: Parameter "week" description should start with a capital letter
pandas.tseries.offsets.WeekOfMonth: Parameter "weekday" description should start with a capital letter
pandas.tseries.offsets.LastWeekOfMonth: Parameter "weekday" description should start with a capital letter
pandas.tseries.offsets.FY5253: Parameter "weekday" description should start with a capital letter
pandas.tseries.offsets.FY5253: Parameter "variation" description should start with a capital letter
pandas.tseries.offsets.FY5253Quarter: Parameter "weekday" description should start with a capital letter
pandas.tseries.offsets.FY5253Quarter: Parameter "qtr_with_extra_week" description should start with a capital letter
pandas.tseries.offsets.FY5253Quarter: Parameter "variation" description should start with a capital letter
pandas.tseries.offsets.CDay: Parameter "weekmask" description should start with a capital letter
pandas.tseries.offsets.CDay: Parameter "holidays" description should start with a capital letter
```

- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Will continue to work through all PR08 docstring errors.