[1.6.x] Fixed typo in previous commit; refs #21490.

Backport of 03bc0a8 from master

docs/ref/contrib/admin/index.txt

@@ -2354,7 +2354,7 @@ that instance as a ``current_app`` hint to the reverse call. For example,
 if you specifically wanted the admin view from the admin instance named
 ``custom``, you would need to call::
 
-    >>> change_url = urlresolvers.reverse('custom:polls_choice_change',
+    >>> change_url = urlresolvers.reverse('admin:polls_choice_change',
     ...                                   args=(c.id,), current_app='custom')
 
 For more details, see the documentation on :ref:`reversing namespaced URLs