docs: add missing ->CSP

Author: kenjis

user_guide_src/source/outgoing/response/013.php

@@ -1,6 +1,6 @@
 <?php
 
-$response->addChildSrc('https://youtube.com'); // allowed
-$response->reportOnly(true);
-$response->addChildSrc('https://metube.com'); // allowed but reported
-$response->addChildSrc('https://ourtube.com', false); // allowed
+$this->response->CSP->addChildSrc('https://youtube.com'); // allowed
+$this->response->CSP->reportOnly(true);
+$this->response->CSP->addChildSrc('https://metube.com'); // allowed but reported
+$this->response->CSP->addChildSrc('https://ourtube.com', false); // allowed