Replace broken link in G56 (#3702)

The page this links no longer exists.

---------

Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>

Author: hidde-logius

techniques/general/G56.html

@@ -78,12 +78,12 @@ <h4>Visual example of the failure</h4>
             <li>#5 is true</li>
          </ul>
       </section>
-   </section><section id="resources"><h2>Resources</h2>
-      
+    </section><section id="resources"><h2>Resources</h2>
+
          <ul>
             <li>
-               <a href="http://www.eramp.com/david/audio_contrast_general_techs.htm">Audio creation / contrast tutorial</a>
+               <a href="https://web.archive.org/web/20170716020055/http://www.eramp.com/david/audio_contrast_general_techs.htm">Audio creation / contrast tutorial</a>
             </li>
          </ul>
-      
+
    </section></body></html>