ocaml-collision: fix README MDX usage example
The example referenced an unbound [cdm] value and [Cdm] / [Fmt]
modules that weren't in the mdx libraries. Wrap the snippet as
[let report_pc (cdm : Cdm.t) = ...] and add [cdm] / [fmt] to the
mdx stanza so the example typechecks against the real API.