docs(guide/forms): make required ngModel optional in custom e-mail RegExp example

Author: gkalpak

docs/content/guide/forms.ngdoc

@@ -440,8 +440,7 @@ Note that you can alternatively use `ng-pattern` to further restrict the validat
       var EMAIL_REGEXP = /^[a-z0-9!#$%&'*+/=?^_`{|}~.-]+@example\.com$/i;
 
       return {
-        require: 'ngModel',
-        restrict: '',
+        require: '?ngModel',
         link: function(scope, elm, attrs, ctrl) {
           // only apply the validator if ngModel is present and Angular has added the email validator
           if (ctrl && ctrl.$validators.email) {