Updated to reflect examples

I saw similar code in examples/ directory that has the master/worker roles swapped from my original fix. The original code is inconsistent and it wasn't clear which was intended as worker vs master, but this new commit matches what's in examples/

docs/running-distributed.rst

@@ -90,26 +90,26 @@ 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 master receives a message of type 'test_users'
+    # Fired when the worker 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 worker receives a message of type 'acknowledge_users'
+    # Fired when the master 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 isinstance(environment.runner, MasterRunner):
+        if not isinstance(environment.runner, MasterRunner):
             environment.runner.register_message('test_users', setup_test_users)
-        if isinstance(environment.runner, WorkerRunner):
+        if not isinstance(environment.runner, WorkerRunner):
             environment.runner.register_message('acknowledge_users', on_acknowledge)
 
     @events.test_start.add_listener
     def on_test_start(environment, **_kwargs):
-        if not isinstance(environment.runner, MasterRunner):
+        if not isinstance(environment.runner, WorkerRunner):
             users = [
                 {"name": "User1"},
                 {"name": "User2"},