Fix video object detection example (#1017)

In main.js, scaleSlider is needed which result into an undefined error

examples/video-object-detection/index.html

@@ -34,10 +34,16 @@ <h4>
       <br>
       <input id="threshold" type="range" min="0.01" max="1" step="0.01" value="0.25" disabled>
     </div>
+    <div>
+      <label>Image scale</label>
+      (<label id="scale-value">0.5</label>)
+      <br>
+      <input id="scale" type="range" min="0" max="1" step="0.01" value="0.5" disabled>
+    </div>
   </div>
   <label id="status"></label>
 
   <script type="module" src="/main.js"></script>
 </body>
 
-</html>
+</html>