Fix blatant lie in description of colour field

[Alhadis]

A glaring mistake is sitting in plain sight in language.yml's description of the color field:

Only used if type is "programming" or "prose".

This should, of course, say markup, not prose. 😉

[Alhadis]

@lildude I can't merge until CI passes its checks, and the failures are unrelated to this PR:

 1) Failure:
TestGrammars#test_submodules_have_approved_licenses [/home/travis/build/github/linguist/test/test_grammars.rb:115]:
The following submodules have unapproved licenses:
** 68-line list omitted for brevity **

What should I do?

[lildude]

What should I do?

Watch in awe as I whip out my admin rights and click the button...

[Alhadis]

HAHA, you showoff. 😆

Thanks! 👍