Fix doc error (#647)

Author: Ross Mechanic

docs/querying_history.rst

@@ -166,5 +166,6 @@ To filter changes to the data, a relationship to the history can be established.
 You can also prefetch the objects with this relationship using somthing like this for example to prefetch order by history_date descending:
 
 .. code-block:: python
+
     Poll.objects.filter(something).prefetch_related(Prefetch('history', queryset=Poll.history.order_by('-history_date'),
                                                 to_attr='ordered_histories')