DOC: Fix Docstring for DataFrame nlargest `keep` option (#44040)
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] Add double backticks for DataFrame nlargest `keep` option to keep the style consistent

From
https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.nlargest.html
![image](https://user-images.githubusercontent.com/25895405/137416950-97b0db11-382f-410f-ac90-50ef2b25c32b.png)
To
![image](https://user-images.githubusercontent.com/25895405/137420535-275670da-094c-4533-9eac-82a36f52d5f4.png)

