ocaml-rice: rewrite README example to typecheck
The block had a stray [in] after [Bytes.make 1024 '\x00'] and used a
top-level [match] expression. Promote to top-level [let], handle
[decompress] via [let () = match ... with | Ok ...] + assert, switch
[Printf.printf]/[Printf.eprintf] to [assert]/[Fmt.failwith], and add
[fmt] to the mdx libraries.