DOC: uses black to fix formatting #36777
Fixes /docs/source/user_guide/merging.rst formatting. Passes flake8-rst test.

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

ref https://github.com/pandas-dev/pandas/issues/36777