DOC: A blank line is needed at the beginning of list.
Fixes not displaying as a list in keep parameter of ``DataFrame.drop_duplicates``.
Added a blank line.

![image](https://user-images.githubusercontent.com/30496251/189386107-1421391f-22bf-461f-9cd2-0f8d87471272.png)

Yes!
![image](https://user-images.githubusercontent.com/30496251/189389959-5756fca3-fd60-45ad-ab91-73955e0a8751.png)
 Thanks @15b3 