docs(guide): change prototype methods to scope methods in DI examples

As explained in 'Understanding the Controller Component', Controllers
written for new (post 1.0 RC) versions of Angular need to add methods to
the scope directly, not the function's prototype. Correcting this
example should remove any ambiguity, especially for beginners.

docs/content/guide/di.ngdoc

@@ -196,14 +196,13 @@ Controllers are classes which are responsible for application behavior. The reco
 declaring controllers is:
 
 <pre>
-  var MyController = function(dep1, dep2) {
-    ...
-  }
-  MyController.$inject = ['dep1', 'dep2'];
-  
-  MyController.prototype.aMethod = function() {
+  var MyController = function($scope, dep1, dep2) {
     ...
+    $scope.aMethod = function() {
+      ...
+    }
   }
+  MyController.$inject = ['$scope', 'dep1', 'dep2'];
 </pre>