Updated docs to match reality, where drafts are dated based on file modification time

See https://github.com/mojombo/jekyll/blob/8e7b6bf5ff56e6bb8d0f1019c19b351f1d600e39/lib/jekyll/draft.rb#L28 -- source uses `File.mtime` on the draft file, not `Time.now` as the doc says.