Missing class imports

docs/reference/hooks.rst

@@ -624,6 +624,8 @@ Hooks for customising the way admins are directed through the process of editing
 
   .. code-block:: python
 
+    from django.http import HttpResponse
+
     from wagtail.core import hooks
 
     from .models import AwesomePage
@@ -852,6 +854,8 @@ Page serving
 
   .. code-block:: python
 
+    from django.http import HttpResponse
+
     from wagtail.core import hooks
 
     @hooks.register('before_serve_page')