Update module.ngdoc Hello World example

Helpful for people new to Angular to see the ng-app declaration in context with the expression example. This will help illustrate the "Important thing to notice" point which follows: "The reference to myApp module in <html ng-app="myApp">. This is what bootstraps the app using your module."
we can't really use the `html` tag for this, because the example is already inside an `html` tag :(