form_for, and form_tag, have been soft-deprecated in favour of
form_with. However, the documentation outside of the Rails Guide
does not show this change often missing form_with completely.

This PR fixes that.

Changes:
- Adding form_with before form_for in text.
- Replacing form_for with form_with in example code.

Co-authored-by: Hartley McGuire <skipkayhil@gmail.com>