[examples/index.html]: remove onmousewheel (#21647)

* [examples/index.html]: Change the onmousewheel of the viewer

* [examples/index.html]: remove onmousewheel

Author: puxiao

examples/index.html

@@ -38,7 +38,7 @@ <h1><a href="https://threejs.org">three.js</a></h1>
 
 		</div>
 
-		<iframe id="viewer" name="viewer" allowfullscreen allowvr onmousewheel=""></iframe>
+		<iframe id="viewer" name="viewer" allowfullscreen allowvr></iframe>
 
 		<a id="button" target="_blank"><img src="../files/ic_code_black_24dp.svg"></a>