Add marker

Co-authored-by: Kamil Tekiela <[email protected]>

Author: Crell

language/oop5/visibility.xml

@@ -123,7 +123,7 @@ $b->pubYear = 2023; // Fatal Error
      </programlisting>
     </example>
     <simpara>
-     As of PHP 8.5, set visibility may also be applied to static properties of classes.
+     As of PHP 8.5, <literal>set</literal> visibility may also be applied to static properties of classes.
     </simpara>
     <example>
      <title>Asymmetric Property visibility</title>