README about creating keys seems wrong

[talex5]

The example in the README says to use:

let lang_key =
  let doc = Key.Doc.create
      ~doc:"The default language for the application." [ "l" ; "lang" ]
  in
  Key.create ~doc ~stage:`Both ~default:"en" "language" Key.Desc.string

It's not clear what Key is here. Maybe it is Mirage.Key or Functoria_key? But neither has a submodule called Doc.