fix(docs/customization): Add models import to fix example (#5520)

CONTRIBUTORS.rst

@@ -388,6 +388,7 @@ Contributors
 * John Carter
 * Jonathan Liuti
 * Rahmi Pruitt
+* Sanyam Khurana
 
 Translators
 ===========

docs/advanced_topics/customisation/page_editing_interface.rst

@@ -159,6 +159,7 @@ or to add custom validation logic for your models:
 .. code-block:: python
 
     from django import forms
+    from django.db import models
     import geocoder  # not in Wagtail, for example only - http://geocoder.readthedocs.io/
     from wagtail.admin.edit_handlers import FieldPanel
     from wagtail.admin.forms import WagtailAdminPageForm