docs: replace title Event with Hooks

This doc is for CI3 users.

Author: kenjis

user_guide_src/source/installation/upgrade_4xx.rst

@@ -135,8 +135,8 @@ Helpers
     - `redirect() Documentation CodeIgniter 3.X <https://codeigniter.com/userguide3/helpers/url_helper.html#redirect>`_
     - `redirect() Documentation CodeIgniter 4.X <../general/common_functions.html#redirect>`_
 
-Events
-======
+Hooks
+=====
 
 - `Hooks <https://www.codeigniter.com/userguide3/general/hooks.html>`_ have been
   replaced by :doc:`../extending/events`.