fix three.js link

threejs/lessons/toc.html

@@ -17,6 +17,6 @@
 </ul>
 <ul>
   <li><a href="https://github.com/greggman/threejsfundamentals">github</a></li>
-  <li><a href="https://three.js">three.js</a></li>
+  <li><a href="https://threejs.org">three.js</a></li>
   <li><a href="https://threejs.org/docs/">three.js docs</a></li>
 </ul>