[Forms] Fix submit button usage in template

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3
| Fixed tickets | -

In the controller above the form was built with the new "submit" field type, so the submit button should be rendered with the "form_widget()" Twig function following the field type reference. Otherwise the "form_end()" function will render it in addition to the plain HTML "input[type=submit]" button.