[examples/index.html]: remove onmousewheel

[puxiao]

The first reason

mousewheel

https://developer.mozilla.org/en-US/docs/Web/API/Element/mousewheel_event

This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards ...

Important: Instead of this obsolete event, use the standard wheel event.

Second reason

oh...

So

-	<iframe id="viewer" name="viewer" allowfullscreen allowvr onmousewheel=""></iframe>
+	<iframe id="viewer" name="viewer" allowfullscreen allowvr></iframe>

...

// Events

+	viewer.addEventListener( 'wheel', function ( event ) {
+
+		event.preventDefault();
+
+	}, { passive: false } );

This contribution is funded by Example: https://raw.githack.com/puxiao/three.js/dev/examples/index.html

[mrdoob]

Thanks!