ocaml-turbo: rewrite README example and fix rate variants
The block had a top-level [match] expression and used [`One_third],
but the [rate] type uses [`R1_3] / [`R1_2] / [`R1_4] / [`R1_6]. Wrap
as [roundtrip data = ...], assert [Bytes.equal] on the decoded data,
and fix the rate constants in the example and the bullet list.