Fix write typespec to handle lists of strings

[sax]

I found that the following code would give me a dialyzer error that my function would never return.

    Mudbrick.new(compress: true, fonts: %{bodoni: Extra.Font.bodoni_regular()})
    |> Mudbrick.page(size: Mudbrick.Page.size(:a4))
    |> Mudbrick.text(
      [
        "Name: ",
        "Stuff:\n"
      ],
      auto_kern: true,
      font: :bodoni,
      font_size: 12,
      leading: 18,
      position: {26, height - 48}
    )
    |> Mudbrick.render()

[camelpunch]

Thank you!