RPC server doc: use the blocking executor

The eventlet executor need additional code to make the RPC server sample
work. Using the blocking executor makes things easier to setup for a
newcomer.

Change-Id: I56dfa54a178962d4ced34e2f15eaf9312b7087cc

oslo/messaging/rpc/server.py

@@ -68,7 +68,7 @@ A simple example of an RPC server with multiple endpoints might be::
         TestEndpoint(),
     ]
     server = messaging.get_rpc_server(transport, target, endpoints,
-                                      executor='eventlet')
+                                      executor='blocking')
     server.start()
     server.wait()