Fix v1 protocol authentication example

Relates to #128

docs/security.rst

@@ -42,15 +42,18 @@ Protocol v1 Authentication
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 When working with Cassandra 1.2 (or a higher version with
 :attr:`~.Cluster.protocol_version` set to ``1``), you will not pass in
-an :class:`~.AuthProvider` instance.  Instead, you should pass a dict
-of credentials with a ``username`` and ``password`` key:
+an :class:`~.AuthProvider` instance.  Instead, you should pass in a
+function that takes one argument, the IP address of a host, and returns
+a dict of credentials with a ``username`` and ``password`` key:
 
 .. code-block:: python
 
     from cassandra.cluster import Cluster
 
-    credentials = {'username': 'joe', 'password': '1234'}
-    cluster = Cluster(auth_provider=credentials, protocol_version=1)
+    def get_credentials(host_address):
+        return {'username': 'joe', 'password': '1234'}
+
+    cluster = Cluster(auth_provider=get_credentials, protocol_version=1)
 
 SSL
 ---