Fix rpc client docs

Missing a colon before the example code, see:

  http://docs.openstack.org/developer/oslo.messaging/rpcclient.html

Change-Id: Icd875e509b0057c2c739d43c11cd37ab923b59ee

oslo/messaging/rpc/client.py

@@ -236,7 +236,7 @@ class RPCClient(object):
             return cctxt.call(ctxt, 'test', arg=arg)
 
     However, this class can be used directly without wrapping it another class.
-    For example:
+    For example::
 
         transport = messaging.get_transport(cfg.CONF)
         target = messaging.Target(topic='testtopic', version='2.0')