Fixes to introduction

Perhaps move this to the end of the previous paragraph. So:

> The first thing any `gfx-rs` program needs to do is get a window to render to. Right now, only the `glfw` library is supported. We create an 800x600 window using the latest OpenGL version that the graphics driver supports:
>
> `...`
>
> The next thing we need to do is create the renderer and device:
>
> `...`