Update advanced_features.rst (#3019)

Author: BackflipPenguin

docs/advanced_features.rst

@@ -346,7 +346,7 @@ running.
 
 The third option runs an event loop in a separate thread.
 pubsub.run_in_thread() creates a new thread and starts the event loop.
-The thread object is returned to the caller of [un_in_thread(). The
+The thread object is returned to the caller of run_in_thread(). The
 caller can use the thread.stop() method to shut down the event loop and
 thread. Behind the scenes, this is simply a wrapper around get_message()
 that runs in a separate thread, essentially creating a tiny non-blocking