[docs] Fixes bad examples in monitoring guide. Closes #1385

docs/userguide/monitoring.rst

@@ -546,24 +546,31 @@ Combining these you can easily process events in real-time:
 
 .. code-block:: python
 
-
     from celery import Celery
 
 
-    def monitor_events(app):
+    def my_monitor(app):
         state = app.events.State()
 
-        def on_event(event):
-            state.event(event)   # <-- updates in-memory cluster state
+        def announce_failed_tasks(event):
+            state.event(event)
+            # task name is sent only with -received event, and state
+            # will keep track of this for us.
+            task = state.tasks.get(event['uuid'])
 
-            print('Workers online: %r' % ', '.join(
-                worker for worker in state.workers if worker.alive
-            ))
+            print('TASK FAILED: %s[%s] %s' % (
+                task.name, task.uuid, task.info(), ))
 
         with app.connection() as connection:
-            recv = app.events.Receiver(connection, handlers={'*': on_event})
+            recv = app.events.Receiver(connection, handlers={
+                    'task-failed': announce_failed_tasks,
+                    '*': state.event,
+            })
             recv.capture(limit=None, timeout=None, wakeup=True)
 
+    if __name__ == '__main__':
+        celery = Celery(broker='amqp://guest@localhost//')
+        my_monitor(celery)
 
 .. note::