add primitives to toc

threejs/lessons/toc.html

@@ -3,6 +3,7 @@
   <ul>
     <li><a href="/threejs/lessons/threejs-fundamentals.html">Three.js Fundamentals</a></li>
     <li><a href="/threejs/lessons/threejs-responsive.html">Three.js Responsive Design</a></li>
+    <li><a href="/threejs/lessons/threejs-primitives.html">Three.js Primitives</a></li>
   </ul>
 </ul>
 <ul>