Update index.html

sandbox/index.html

@@ -32,6 +32,7 @@
     <script src="https://preview.babylonjs.com/cannon.js"></script>
     <script src="https://preview.babylonjs.com/Oimo.js"></script>
     <script src="https://preview.babylonjs.com/babylon.js"></script>
+    <script src="https://preview.babylonjs.com/gui/babylon.gui.min.js"></script>
     <script src="https://preview.babylonjs.com/inspector/babylon.inspector.bundle.js"></script>
 
     <script src="https://preview.babylonjs.com/loaders/babylonjs.loaders.min.js"></script>