feat(docs): Use $scope instead of scope

To avoid confusion I guess it would be good to keep the examples consistent.
While `scope` in `DialogController` will be the same as `$scope` it brings confusion to the reader.
The fix also includes missing `}` mentioned here:

Here's a working pen: http://codepen.io/anon/pen/ZGWOpZ.

Fixes #2788. Closes #2836.

src/components/dialog/dialog.js

@@ -128,14 +128,14 @@ function MdDialogDirective($$rAF, $mdTheming) {
  *          },
  *          controller: DialogController
  *       });
- *       function DialogController(scope, $mdDialog, items) {
- *         scope.items = items;
- *         scope.closeDialog = function() {
+ *       function DialogController($scope, $mdDialog, items) {
+ *         $scope.items = items;
+ *         $scope.closeDialog = function() {
  *           $mdDialog.hide();
  *         }
  *       }
  *     }
- *
+ *   }
  * })(angular);
  * </hljs>
  *