updated indx

Author: Tsung-Wei Huang

index.html

@@ -87,7 +87,7 @@ <h1><b>Taskflow </b><img src="images/logo.png" alt="Logo" style="width:2.5rem;">
 
         <p><a class="btn btn-primary btn-lg mr-2 mt-2" href="https://github.com/taskflow/taskflow" role="button">GitHub &raquo;</a>
         <a class="btn btn-info btn-lg mr-2 mt-2" href="https://taskflow.github.io/taskflow/index.html" role="button">Handbook &raquo;</a>
-        <a class="btn btn-warning btn-lg mr-2 mt-2" href="https://taskflow.github.io/taskflow/release-3-8-0.html" role="button">Releases (latest v3.8) &raquo;</a></p>
+        <a class="btn btn-warning btn-lg mr-2 mt-2" href="https://taskflow.github.io/taskflow/release-3-9-0.html" role="button">Releases (latest v3.9) &raquo;</a></p>
       </div>
     </div>