Fix typo.

Incoporate feedback from @dhermes.

Author: tseaver

gcloud/datastore/query.py

@@ -318,8 +318,8 @@ def fetch(self, limit=None):
   def cursor(self):
     """Returns cursor ID
 
-    .. Caution:: Invoking this method on a query that has not yet has been
-      executed will raise a RuntimeError.
+    .. Caution:: Invoking this method on a query that has not yet been
+       executed will raise a RuntimeError.
 
     :rtype: string
     :returns: base64-encoded cursor ID string denoting the last position