docs(guide/Forms): fix indentation.

Closes #12988

Author: koyner

docs/content/guide/forms.ngdoc

@@ -383,7 +383,7 @@ In the following example we create two directives:
       return {
         require: 'ngModel',
         link: function(scope, elm, attrs, ctrl) {
-        var usernames = ['Jim', 'John', 'Jill', 'Jackie'];
+          var usernames = ['Jim', 'John', 'Jill', 'Jackie'];
 
           ctrl.$asyncValidators.username = function(modelValue, viewValue) {