DOC: fix rsplit doc
- [x] closes #46488 (Replace xxxx with the Github issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] 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.

Gets rid of the `regex` argument in `.str.rsplit`. The docs for `.str.split` remains unchanged

<img width="954" alt="Screenshot 2022-04-22 at 14 07 54" src="https://user-images.githubusercontent.com/15383974/164720328-0375c2d1-ce65-45d1-81f6-968c5869f059.png">

<img width="882" alt="Screenshot 2022-04-22 at 14 10 55" src="https://user-images.githubusercontent.com/15383974/164720818-d202d4eb-fa27-4fb6-8af4-a0e3151fde0a.png">


Hello @dykim29! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2022-04-23 08:02:59 UTC