sync: fix sync::broadcast::Sender<T>::closed() doctest (#7090)

The test's previous iteration could sometimes flake since we didn't
await the completion of the first task. Since the tasks only existed to
`move` the relevant `rx`'s in, to force a drop, we can omit them
entirely and drop the `rx`s via `drop()`. This prevents any
scheduling-related flakes.

tokio/src/sync/broadcast.rs

@@ -825,17 +825,14 @@ impl<T> Sender<T> {
     ///     let (tx, mut rx1) = broadcast::channel::<u32>(16);
     ///     let mut rx2 = tx.subscribe();
     ///
-    ///     tokio::spawn(async move {
-    ///         assert_eq!(rx1.recv().await.unwrap(), 10);
-    ///     });
-    ///
     ///     let _ = tx.send(10);
-    ///     assert!(tx.closed().now_or_never().is_none());
     ///
-    ///     let _ = tokio::spawn(async move {
-    ///         assert_eq!(rx2.recv().await.unwrap(), 10);
-    ///     }).await;
+    ///     assert_eq!(rx1.recv().await.unwrap(), 10);
+    ///     drop(rx1);
+    ///     assert!(tx.closed().now_or_never().is_none());
     ///
+    ///     assert_eq!(rx2.recv().await.unwrap(), 10);
+    ///     drop(rx2);
     ///     assert!(tx.closed().now_or_never().is_some());
     /// }
     /// ```