improve 'Nightly builds' link to cover both 3 and 2

Author: SethTisue

_includes/download-resource-list.html

@@ -55,7 +55,7 @@ <h2 id="other-releases">Other Releases</h2>
       <li><a href="/download/{{ release.version }}.html">Scala {{ release.version }}</a> - Released on {{ release.release_date }}</li>
     </ul></li>
   {% endfor %}
-  <li><a href="https://scala-lang.org/files/archive/nightly/">Nightly builds</a></li>
+  <li><a href="https://docs.scala-lang.org/overviews/core/nightlies.html">Nightly builds</a></li>
   <li><a href="/download/all.html">All Scala releases</a></li>
 </ul>
 </div>