[documentation] [url_launcher] fix for readme code sample (flutter#3308)

* code sample fixed

* updated CHANGELOG.md and pubspec.yaml

Author: Francesco Iapicca

packages/url_launcher/url_launcher/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 5.7.12
+
+* Updated code sample in `README.md`
+
 ## 5.7.11
 
 * Update integration test examples to use `testWidgets` instead of `test`.

packages/url_launcher/url_launcher/README.md

@@ -14,26 +14,23 @@ To use this plugin, add `url_launcher` as a [dependency in your pubspec.yaml fil
 import 'package:flutter/material.dart';
 import 'package:url_launcher/url_launcher.dart';
 
-void main() {
-  runApp(Scaffold(
-    body: Center(
-      child: RaisedButton(
-        onPressed: _launchURL,
-        child: Text('Show Flutter homepage'),
+const _url = 'https://flutter.dev';
+
+void main() => runApp(
+      const MaterialApp(
+        home: Material(
+          child: Center(
+            child: RaisedButton(
+              onPressed: _launchURL,
+              child: Text('Show Flutter homepage'),
+            ),
+          ),
+        ),
       ),
-    ),
-  ));
-}
-
-_launchURL() async {
-  const url = 'https://flutter.dev';
-  if (await canLaunch(url)) {
-    await launch(url);
-  } else {
-    throw 'Could not launch $url';
-  }
-}
+    );
 
+void _launchURL() async =>
+    await canLaunch(_url) ? await launch(_url) : throw 'Could not launch $_url';
 ```
 
 ## Supported URL schemes

packages/url_launcher/url_launcher/pubspec.yaml

@@ -2,7 +2,7 @@ name: url_launcher
 description: Flutter plugin for launching a URL on Android and iOS. Supports
   web, phone, SMS, and email schemes.
 homepage: https://github.com/flutter/plugins/tree/master/packages/url_launcher/url_launcher
-version: 5.7.11
+version: 5.7.12
 
 flutter:
   plugin: