DOC PR09 Add "." on to_hdf format parameter
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

From #28602:
```
pandas.Series.to_hdf: Parameter "format" description should finish with "."
pandas.DataFrame.to_hdf: Parameter "format" description should finish with "."
```