ocaml-xtce: rewrite README example to typecheck
The block referenced [xml_string] free and iterated over a [(name,
param)] tuple, but [Xtce.parameters] is a [parameter list] (each
record has both [name] and [parameter_type_ref] fields). Wrap as
[report xml_string = ...] and switch [Printf.printf] to [Fmt.pr]
with [fmt] in the mdx libs.