BUG: Styling user guide points to a wrong nbviewer link
Just missing the 'user_guide' part => one line change.

- [ ] closes #xxxx   (NOT THAT I KNOW)
- [ ] tests added / passed   (NO CODE CHANGE)
- [ ] passes `black pandas`   (NO CODE CHANGE)
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`   (NO CODE CHANGE)
- [ ] whatsnew entry   (NO CHANGE)
