Fix MIME type request in sample code. Fixes #3292

rawgithub.com and raw.github.com serve files with different MIME types.  Chrome doesn't treat the MIME type served from raw.github.com as executable JavaScript, so the sample code provided in Creating-a-scene.html fails.  This patch changes the requested URL to make the sample code run correctly in all browsers.
Merge remote-tracking branch 'jeremyckahn/patch-1' into dev
Docs: Probably better to encourage including the lib locally instead.