Fixed the documentation for pandas.DataFrame.set_index inplace parameter
Fixed the documentation for `pandas.DataFrame.set_index` 's `inplace` parameter.
Added proper explanation for True case.

Hello @PurushothamanSrikanth! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-11-13 16:08:20 UTC @arw2019 @pep8speaks Is it fine now? thanks @PurushothamanSrikanth 