update toc

threejs/lessons/toc.html

@@ -20,6 +20,7 @@
   <li>Tips</li>
   <ul>
     <li><a href="/threejs/lessons/threejs-debugging-javascript.html">Debugging JavaScript</a></li>
+    <li><a href="/threejs/lessons/threejs-render-on-demand.html">Rendering On Demand</a></li>
   </ul>
   <li>Fundamentals</li>
   <ul>