DOC: fix formatting in .at docstring
- [x] Fix for 9th bullet point of https://github.com/noatamir/pyladies-aug23-sprint/issues/13
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

Done during the 'Let's contribute to pandas - mentored workshop' event - PyLadies Berlin

@noatamir
@phofl 

/preview No preview found for PR #54730. Did the docs build complete?