Added missing reference to custom template tag docs.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16173 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/howto/custom-template-tags.txt

@@ -624,6 +624,8 @@ for example::
 Variable resolution will throw a ``VariableDoesNotExist`` exception if it cannot
 resolve the string passed to it in the current context of the page.
 
+.. _howto-custom-template-tags-simple-tags:
+
 Shortcut for simple tags
 ~~~~~~~~~~~~~~~~~~~~~~~~