Remove mentions of color.ui = auto

This has been the default since v1.8.4.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Merge commit 'refs/pull/118/head' of github.com:github/training-kit into color-auto-fix-118
Merge pull request #125 from github/color-auto-fix-118

Remove `color.ui auto` from materials and closes #118