docs: update header level

Author: kenjis

user_guide_src/source/changelogs/v4.1.6.rst

@@ -10,7 +10,7 @@ Release Date: Not released
     :depth: 2
 
 SECURITY
-========
+********
 
 - *Deserialization of Untrusted Data* found in the ``old()`` function was fixed. See the `Security advisory <https://github.com/codeigniter4/CodeIgniter4/security/advisories/GHSA-w6jr-wj64-mc9x>`_ for more information.