error using example provided in readme

[EdOverride]

Main readme has this example:

require 'github/markup'

GitHub::Markup.render_s(GitHub::Markups::MARKUP_MARKDOWN, "* One\n* Two")

But the value that GitHub::Markups::MARKUP_MARKDOWN returns is :markdown, however in the renderer (https://github.com/github/markup/blob/master/lib/github/markup/markdown.rb) there is no such value, hence error no suitable markdown gem found is raised.

I think the main readme should be updated.