DOC: minor documentation fix on frame.dropna
- [x] closes #21209 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

After this PR, it should look like this:
<img width="749" alt="pr" src="https://user-images.githubusercontent.com/14131823/40563992-e1536170-605e-11e8-8fd7-13bc1e0981d0.png">
