docs: fix 404 link and make the note more accurate

Author: kenjis

user_guide_src/source/installation/upgrade_security.rst

@@ -13,7 +13,7 @@ Documentations
 - :doc:`Security Documentation Codeigniter 4.X </libraries/security>`
 
 .. note::
-    If you use the `form helper </user_guide/helpers/form_helper.html>`_, then form_open() will automatically insert a hidden CSRF field in your forms. So you do not have to upgrade this by yourself.
+    If you use the :doc:`form helper </helpers/form_helper>` and enable the CSRF filter globally, then ``form_open()`` will automatically insert a hidden CSRF field in your forms. So you do not have to upgrade this by yourself.
 
 What has been changed
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