docs: fix comment in sample code

Author: kenjis

user_guide_src/source/libraries/validation/008.php

@@ -4,7 +4,8 @@
 $request    = \Config\Services::request();
 
 if ($validation->withRequest($request)->run()) {
-    // If you want to get the validated data.
+    // If you use the input data, you should get it from the getValidated() method.
+    // Otherwise you may create a vulnerability.
     $validData = $validation->getValidated();
 
     // ...