DOC: fix docstring for DataFrame.round()
- [ ] closes #60009
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-

Since I'm a beginner, I just simply fixed the docstring. If my work is correct, I'll continue fixing the 'See Also' and 'Examples' sections of DataFrame.round() and Series.round() Thanks @eightyseven 