Fix broken docs for Query.add_filter

Missing colon prevented the rST from rendering properly.

gcloud/datastore/query.py

@@ -207,7 +207,7 @@ def add_filter(self, property_name, operator, value):
 
         :type value: :class:`int`, :class:`str`, :class:`bool`,
                      :class:`float`, :class:`NoneType`,
-                     :class`datetime.datetime`
+                     :class:`datetime.datetime`
         :param value: The value to filter on.
 
         :raises: :class:`ValueError` if ``operation`` is not one of the