message: Fix README example to updated color tuple struct
  This was missed by CI integration since both PRs were open in the same
  timespan but the build job was not restarted after the merge of the
  struct rework. This would have to be done manually.
diff: @@ -241,7 +241,7 @@ fn main() {
  }
  let pixel = imgbuf.get_pixel_mut(x, y);
  - let data = (*pixel as image::Rgb<u8>).data;
  + let image::Rgb(data) = *pixel;
  *pixel = image::Rgb([data[0], i as u8, data[2]]);
  }
  }
  