DOC: Fix SS03 docstring error
Fixes SS03 errors. 

```
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
```

Related to #27977, #30733 cc @datapythonista

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes black pandas
- [x] passes git diff upstream/master -u -- "*.py" | flake8 --diff
- [ ] whatsnew entry

When I ran the script, there are others left, but they might be false positives since I can't find them in the codebase.