Change "pls" to "please" in warning message
 - [ ] closes #xxxx
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [x] whatsnew entry

Code is now more formal when talking to users, as to not disrespect them!
Full warning:
`"by argument to sort_index is deprecated, please use .sort_values(by=...)"`