Add missing newline
The line ">>> idx.isin([1, 4]) array([ True, False, False])" is was not rendered properly, because it was missing a newline above it.

https://pandas.pydata.org/docs/reference/api/pandas.Index.isin.html

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
