fix broken link

Author: Fienne

src/_templates/layout.html

@@ -15,7 +15,7 @@
           <a class="nav-link" href="https://github.com/common-workflow-language/user_guide/">Source</a>
         </li>
         <li class="nav-item">
-          <a class="nav-link" href="https://github.com/common-workflow-language/user_guide/blob/main/CITATION">Cite</a>
+          <a class="nav-link" href="https://github.com/common-workflow-language/user_guide/blob/main/CITATION.md">Cite</a>
         </li>
       </ul>
     </div>