DOC: Fixed PR09 docstring errors in pandas.tseries (#27977)
- [ ] xref #27977
- [ ] tests added / passed
- [ ] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x ] fixed minor issues in the couple of docstrings (ErrorCode: PR09) in pandas.tseries.offsets.py, namely added dots at the end of parameters {param_name} descriptions. Some of the ErrorCodes: PR09 still appear when running the validation script. This is due the script not covering some of the special cases (such as extra info in the bullet points format in the descriptions of parameters {param_name}), see #25786 and #25461.

Thanks a lot for the contribution! The failing CI are related to https://github.com/pandas-dev/pandas/issues/28708, and the doc and checks CI builds are passing, so going to merge this.