Update related resource section in Understanding SC 3.2.5 (superseded RFC) (#4302)

Replace RFC 2616 with RFC 9110.

Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>
Co-authored-by: Mike Gower <mikegower@gmail.com>

Author: 3 people

understanding/20/change-on-request.html

@@ -125,7 +125,7 @@ <h2>Resources for Change on Request</h2>
 
          <li> 
 
-            <a href="https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3">HTTP/1.1 Status Code Definitions: Redirection 3xx</a>.
+            <a href="https://www.rfc-editor.org/rfc/rfc9110#name-redirection-3xx">RFC 9110: HTTP Semantics 15.4. Redirection 3xx</a>.
 
          </li>