Update references from deprecated GLFW 2 to current GLFW 3.

GLFW 2 has been deprecated, and by now GLFW 3 is better in every way. There is absolutely no reason to point people to the older version.

On a separate note, I'm not even sure if this site is necessary or how many views it gets. It is certainly not kept up to date, so I would vote to remove it if others also feel it's not useful to have it.