Update README.md

Try to clarify initial execution.

packages/flutter/README.md

@@ -39,7 +39,7 @@ void main() {
 }
 ```
 
-Execution starts in `main`, which runs a new instance of the `HelloWorldApp`.
+Execution starts in `main`, which in this example runs a new instance of the `HelloWorldApp`.
 The `HelloWorldApp` builds a `Text` widget containing the famous `Hello, world!`
 string and centers it on the screen using a `Center` widget. To learn more about
 the widget system, please see the [widgets tutorial](lib/widgets/README.md).