Update step_02.ngdoc

The changes suggested above put things in a better sequence and in terms that are more easily understood by a new-comer.  Firstly, it is best to introduce the concept of "Scope" before talking about it.  That's why I have moved it up.

Next, with the present writing I found myself lost for a moment.  Because of my background with the Play Framework I went looking for a folder called controllers.  In the Play Framework, there are three folder within the app folder.  These are views, controllers and models.

Instead, with angularjs, controllers are located in app/js/controllers.js, a file, not a folder.