sync: fix Notify example (#4212)

tokio/src/sync/notify.rs

@@ -57,13 +57,16 @@ type WaitList = LinkedList<Waiter, <Waiter as linked_list::Link>::Target>;
 ///     let notify = Arc::new(Notify::new());
 ///     let notify2 = notify.clone();
 ///
-///     tokio::spawn(async move {
+///     let handle = tokio::spawn(async move {
 ///         notify2.notified().await;
 ///         println!("received notification");
 ///     });
 ///
 ///     println!("sending notification");
 ///     notify.notify_one();
+///
+///     // Wait for task to receive notification.
+///     handle.await.unwrap();
 /// }
 /// ```
 ///