DOC clean up positional-args deprecation warnings in whatsnew
- [ ] 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

Thanks @MarcoGorelli i'll merge this as already an improvement. can address @jbrockmendel comment in a follow-up @MarcoGorelli this hasn't rendered correctly as a list https://pandas.pydata.org/pandas-docs/dev/whatsnew/v1.3.0.html#deprecations