tour: fix pageup/pagedown navigation on non-chrome browsers

FF and IE fail to populate event.keyCode, but instead populate
event.which with the key code. event.key is a DOMString of "PageUp"
or "PageDown" in this case, so remove the invalid comparision to a
numeric key code and instead compare the keycode to event.which.

Fixes golang/go#8829

Change-Id: I7192d2e561e2be5472018d8a3b06a91ab9dab49b
Reviewed-on: https://go-review.googlesource.com/15001
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed