Add missing import to example code snippet (#6638)

Adding this makes the code snippet ready to use via copy and paste.

CONTRIBUTORS.rst

@@ -491,6 +491,7 @@ Contributors
 * Vadim Karpenko
 * Bohreromir
 * Fernando Cordeiro
+* Matthias Rohmer
 
 Translators
 ===========

docs/reference/contrib/forms/index.rst

@@ -28,6 +28,7 @@ Within the ``models.py`` of one of your apps, create a model that extends ``wagt
 
 .. code-block:: python
 
+    from django.db import models
     from modelcluster.fields import ParentalKey
     from wagtail.admin.edit_handlers import (
         FieldPanel, FieldRowPanel,