Added missing import in docs/ref/contrib/admin/index.txt.

Author: Antoliny0919

docs/ref/contrib/admin/index.txt

@@ -2658,6 +2658,7 @@ It is possible to use an inline with generically related objects. Let's say
 you have the following models::
 
     from django.contrib.contenttypes.fields import GenericForeignKey
+    from django.contrib.contenttypes.models import ContentType
     from django.db import models