Getting started guide shows aparently non-existing uniforms fogStart and fogEnd

Tutorial talks about uniforms when showing [how to implement fog](https://github.com/IrisShaders/ShaderDoc/blob/master/getting-started/3-improving.md#uniforms), but [uniforms.md](https://github.com/IrisShaders/ShaderDoc/blob/master/uniforms.md) has no mention of `fogStart` and `fogEnd`