Use atom/language-gfm’s Markdown grammar.

We render [GitHub Flavoured Markdown](https://help.github.com/articles/github-flavored-markdown/) on the site. We can use Atom’s grammar to ensure that syntax highlighting uses the same dialect.
Remove the accidental “m”.
Merge pull request #1864 from github/robrix-patch-1

Use atom/language-gfm’s Markdown grammar.