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
@@ -1069,28 +1069,31 @@ class _AnimatedAlignState extends AnimatedWidgetBaseState<AnimatedAlign> {
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return 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(
-///           onTap: () {
-///             setState(() {
-///               selected = !selected;
-///             });
-///           },
-///           child: Container(
-///             color: Colors.blue,
-///             child: Text('Tap me to start the animated transition'),
+///   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,
+///           duration: Duration(seconds: 2),
+///           curve: Curves.fastOutSlowIn,
+///           child: GestureDetector(
+///             onTap: () {
+///               setState(() {
+///                 selected = !selected;
+///               });
+///             },
+///             child: Container(
+///               color: Colors.blue,
+///               child: Center(child: Text('Tap me')),
+///             ),
 ///           ),
 ///         ),
-///       ),
-///     ],
+///       ],
+///     ),
 ///   );
 /// }
 ///```