Example had inconsistent labeling

The example inconsistently labeled worker and master in the comments and/or registered the wrong messages to the wrong runners.

docs/running-distributed.rst

@@ -90,21 +90,21 @@ order to coordinate data. This can be easily accomplished with custom messages u
     from locust import events
     from locust.runners import MasterRunner, WorkerRunner
 
-    # Fired when the worker receives a message of type 'test_users'
+    # Fired when the master receives a message of type 'test_users'
     def setup_test_users(environment, msg, **kwargs):
         for user in msg.data:
             print(f"User {user['name']} received")
         environment.runner.send_message('acknowledge_users', f"Thanks for the {len(msg.data)} users!")
 
-    # Fired when the master receives a message of type 'acknowledge_users'
+    # Fired when the worker receives a message of type 'acknowledge_users'
     def on_acknowledge(msg, **kwargs):
         print(msg.data)
 
     @events.init.add_listener
     def on_locust_init(environment, **_kwargs):
-        if not isinstance(environment.runner, MasterRunner):
+        if isinstance(environment.runner, MasterRunner):
             environment.runner.register_message('test_users', setup_test_users)
-        if not isinstance(environment.runner, WorkerRunner):
+        if isinstance(environment.runner, WorkerRunner):
             environment.runner.register_message('acknowledge_users', on_acknowledge)
 
     @events.test_start.add_listener