Update df.to_stata() docstring
Updates the docstring for the `to_stata()` method to fix the `dataset_label` parameter. See #19817. Also changes the examples; it's a little weird for the examples for `to_stata()` to detail the `StataWriter` class.

- [ ] closes #19817
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
