DOC: fix RT02 errors in pd.io.formats.style #49968
Simplified return type for several Styler methods in pd.io.formats.style.

- [x] xref #49968
- [ ] tests added and passed
- [x] passes pre-commit code checks
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

I'm happy to add a CI check for RT02, but I'm not sure how to exclude the files that still need doing from #49968. 