typo: pivot_table -> pivot
I'm new to pandas and reading the docs for the first time.

I believe that the reference in the *pivot* section should be to `pivot`, not `pivot_table`, to match the code in the example.

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