DOC: Fix column alignment in df.mul docstring
- [ ] closes #xxxx (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 [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.

Found an alignment issue in the documentation that was due to a few tabs being in there. Changing them to spaces fixes the issue.



**Before this pull request:**
(Screenshot taken from https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.add.html)
<img width="856" alt="image" src="https://user-images.githubusercontent.com/122238526/211251882-8bebd9f1-17bd-41f9-93bb-14e81620981a.png">




**After this pull request:**
(Screenshot taken from docs built locally)
<img width="849" alt="image" src="https://user-images.githubusercontent.com/122238526/211251773-35d73066-5537-4b74-9f6e-d8e750bfcfb4.png">



Hi all - first-timer here.  Thanks for reviewing and for any feedback! thx @smij720 