Clarify sentence.

tseaver · tseaver · commit 8e09bfd11fdd · 2014-10-01T16:51:22.000-04:00
Feedback from @silvolu.
diff --git a/gcloud/datastore/transaction.py b/gcloud/datastore/transaction.py
@@ -21,7 +21,8 @@ class Transaction(object):
     ...   entity1.save()
     ...   entity2.save()
 
-  By default, the transaction is rolled back is an error::
+  By default, the transaction is rolled back if the transaction block
+  exits with an error::
 
     >>> from gcloud import datastore
     >>> dataset = datastore.get_dataset('dataset-id', email, key_path)
