Fix JS reference in index

Author: Matt Calthrop

public/index.html

@@ -19,6 +19,6 @@
     <!--<script type="text/babel" src="scripts/tutorial4-5.js"></script>-->
     <!--<script type="text/babel" src="scripts/tutorial6-7.js"></script>-->
     <!--<script type="text/babel" src="scripts/tutorial8-10.js"></script>-->
-    <script type="text/babel" src="scripts/tutorial11-12.js"></script>
+    <script type="text/babel" src="scripts/tutorial11-13.js"></script>
   </body>
 </html>