Add missing ? operator after handle.await

According to line#118, there should be a `?` operator after `await`.

docs/src/tutorial/receiving_messages.md

@@ -111,7 +111,7 @@ We can "fix" it by waiting for the task to be joined, like this:
 #
 # async move |stream| {
 let handle = task::spawn(connection_loop(stream));
-handle.await
+handle.await?
 # };
 ```