DOC: Improve wording for DataFrame.rename summary 
An attempt to improve the summary for DataFrame.rename.

- [x] closes [this issue](https://github.com/noatamir/pyladies-berlin-sprints/issues/19)
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

@phofl @noatamir 

thx @borovikova 