Refactor Hero example (#75895)

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

@@ -118,7 +118,7 @@ enum HeroFlightDirection {
 ///    return Column(
 ///      crossAxisAlignment: CrossAxisAlignment.start,
 ///      children: <Widget>[
-///        SizedBox(
+///        const SizedBox(
 ///          height: 20.0,
 ///        ),
 ///        ListTile(
@@ -127,7 +127,7 @@ enum HeroFlightDirection {
 ///            child: _blueRectangle(Size(50,50)),
 ///          ),
 ///          onTap: () => _gotoDetailsPage(context),
-///          title: Text('Tap on the icon to view hero animation transition.'),
+///          title: const Text('Tap on the icon to view hero animation transition.'),
 ///        ),
 ///      ],
 ///    );
@@ -145,7 +145,7 @@ enum HeroFlightDirection {
 ///    Navigator.of(context).push(MaterialPageRoute(
 ///      builder: (BuildContext context) => Scaffold(
 ///        appBar: AppBar(
-///          title: Text('second Page'),
+///          title: const Text('second Page'),
 ///        ),
 ///        body: Center(
 ///          child: Column(