Update Draggable YouTube video link (#128078)

The old Draggable video had a mistake in it, we've since uploaded a newer version that's been corrected. Thanks for catching that @gspencergoog!

- [na] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
- [na] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
- [na] I listed at least one issue that this PR fixes in the description above.
- [na] I added new tests to check the change I am making, or this PR is [test-exempt].
- [na] All existing and new tests are passing.

packages/flutter/lib/src/widgets/drag_target.dart

@@ -140,7 +140,7 @@ Offset pointerDragAnchorStrategy(Draggable<Object> draggable, BuildContext conte
 /// [childWhenDragging] when one or more drags are underway. Otherwise, this
 /// widget always displays [child].
 ///
-/// {@youtube 560 315 https://www.youtube.com/watch?v=QzA4c4QHZCY}
+/// {@youtube 560 315 https://www.youtube.com/watch?v=q4x2G_9-Mu0}
 ///
 /// {@tool dartpad}
 /// The following example has a [Draggable] widget along with a [DragTarget]