Update hello world example to match current behavior (#678)

* Update widgets-intro.md

Update hello world example with required textDirection as in examples/hello_world/lib/main.dart

* Update widgets-intro.md

Added space after comma per request and formatting guidelines.

* Reformat with trailing commas

widgets-intro.md

@@ -35,7 +35,14 @@ with a widget:
 import 'package:flutter/material.dart';
 
 void main() {
-  runApp(new Center(child: new Text('Hello, world!')));
+  runApp(
+    new Center(
+      child: new Text(
+        'Hello, world!',
+        textDirection: TextDirection.ltr,
+      ),
+    ),
+  );
 }
 ```