fix dependency for ch10 render hex thread pool

[SteveLauC]

add missing dependency :

• std::thread
• crossbeam::unbounded

refine code according to clippy's cue:

fn generate_svg(path_data: Vec<Command>) -> Document {
  ...
  Document::new()
    .set("viewBox", (0, 0, HEIGHT, WIDTH))
    .set("height", HEIGHT)
    .set("width", WIDTH)
    .set("style", "style=\"outline: 5px solid #800000;\"")
    .add(background)
    .add(sketch)
    .add(border)
}