Added $auth = Yii::$app->authManager; to authorization guide to show where $auth is coming from

docs/guide/security-authorization.md

@@ -342,6 +342,8 @@ The rule above checks if the `post` is created by `$user`. We'll create a specia
 command we've used previously:
 
 ```php
+$auth = Yii::$app->authManager;
+
 // add the rule
 $rule = new \app\rbac\AuthorRule;
 $auth->add($rule);
@@ -444,6 +446,8 @@ class UserGroupRule extends Rule
     }
 }
 
+$auth = Yii::$app->authManager;
+
 $rule = new \app\rbac\UserGroupRule;
 $auth->add($rule);