Add Orderable to list of imported wagtailcore models

docs/topics/snippets.rst

@@ -125,7 +125,7 @@ To attach multiple adverts to a page, the ``SnippetChooserPanel`` can be placed
 
   from django.db import models
 
-  from wagtail.wagtailcore.models import Page
+  from wagtail.wagtailcore.models import Page, Orderable
   from wagtail.wagtailsnippets.edit_handlers import SnippetChooserPanel
 
   from modelcluster.fields import ParentalKey