DOC: correctly format bullets in parameter descriptions for `fillna` methods
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This is a small PR to correct formatting issues in docstrings for `method` options in following `fillna` methods:

- [pandas.Series.fillna](https://pandas.pydata.org/docs/reference/api/pandas.Series.fillna.html)
- [pandas.DataFrame.fillna](https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.fillna.html)
- [pandas.api.extensions.ExtensionArray.fillna](https://pandas.pydata.org/docs/reference/api/pandas.api.extensions.ExtensionArray.fillna.html)

Many thanks.
Great, thanks @noritada 