Fix broken horizon-terminal-* slugs

[tckmn]

The base16-horizon-terminal-dark theme previously referred to base16-horizon-dark instead, and similarly for -light. This is definitely incorrect for the g:colors_name variable, as that should match the name of the colorscheme. I'm not sure whether this was intentional for base16-shell purposes as I don't use that software so haven't tested it, but if not then probably the root cause is an error in whatever .yaml was fed to the template to generate these files (I looked briefly and couldn't find the source of those).