fix: Update 404.html (#468)

Fixes #467

ietf/templates/404.html

@@ -24,7 +24,7 @@
                 <div class="u-max-text-width">
                     <h1>HTTP Status Code: 404 Page Not Found</h1>
 
-                    <p>Did you know that the IETF is where the <a href="https://www.ietf.org/rfc/rfc7231.txt">404 error code</a> is defined?</p>
+                    <p>Did you know that the IETF is where the <a href="https://www.rfc-editor.org/rfc/rfc9110.html">404 error code</a> is defined?</p>
 
                     <p>There was no Web page at the URL provided. Please use the standard site navigation above or the search bar below to find what you’re looking for.</p>
                 </div>