Fix example.

Fixes celery#6459.

Author: Omer Katz

examples/next-steps/proj/celery.py

@@ -2,7 +2,7 @@
 
 app = Celery('proj',
              broker='amqp://',
-             backend='amqp://',
+             backend='rpc://',
              include=['proj.tasks'])
 
 # Optional configuration, see the application user guide.