docs(ngSubmit): reference correct object ($scope) in example.

Switched from this to $scope. It's confusing using $scope and this interchangeably.

Closes #6375

Author: RefuX

src/ng/directive/ngEventDirs.js

@@ -313,9 +313,9 @@ forEach(
           $scope.list = [];
           $scope.text = 'hello';
           $scope.submit = function() {
-            if (this.text) {
-              this.list.push(this.text);
-              this.text = '';
+            if ($scope.text) {
+              $scope.list.push(this.text);
+              $scope.text = '';
             }
           };
         }