Fixed #11137 -- Add missing base class in proxy model extra managers doc. Thanks ekarulf.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10818 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/topics/db/models.txt

@@ -1101,7 +1101,7 @@ manager <custom-managers-and-inheritance>` documentation: create a base class
 containing the new managers and inherit that after the primary base class::
 
     # Create an abstract class for the new manager.
-    class ExtraManagers:
+    class ExtraManagers(models.Model):
         secondary = NewManager()
 
         class Meta: