Center the AnimatedPositioned code sample (#69644)

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

@@ -1058,7 +1058,7 @@ class _AnimatedAlignState extends AnimatedWidgetBaseState<AnimatedAlign> {
 /// it also requires more development overhead as you have to manually manage
 /// the lifecycle of the underlying [AnimationController].
 ///
-/// {@tool dartpad --template=stateful_widget_scaffold}
+/// {@tool dartpad --template=stateful_widget_scaffold_center}
 ///
 /// The following example transitions an AnimatedPositioned
 /// between two states. It adjusts the `height`, `width`, and
@@ -1069,13 +1069,15 @@ class _AnimatedAlignState extends AnimatedWidgetBaseState<AnimatedAlign> {
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return Stack(
+///   return Container(
+///     width: 200,
+///     height: 350,
+///     child: Stack(
 ///       children: [
 ///         AnimatedPositioned(
 ///           width: selected ? 200.0 : 50.0,
 ///           height: selected ? 50.0 : 200.0,
 ///           top: selected ? 50.0 : 150.0,
-///         right: 150.0,
 ///           duration: Duration(seconds: 2),
 ///           curve: Curves.fastOutSlowIn,
 ///           child: GestureDetector(
@@ -1086,11 +1088,12 @@ class _AnimatedAlignState extends AnimatedWidgetBaseState<AnimatedAlign> {
 ///             },
 ///             child: Container(
 ///               color: Colors.blue,
-///             child: Text('Tap me to start the animated transition'),
+///               child: Center(child: Text('Tap me')),
 ///             ),
 ///           ),
 ///         ),
 ///       ],
+///     ),
 ///   );
 /// }
 ///```