ocaml-ocm: defer file I/O via report function
The README example called [Ocm.of_file "satellite.ocm"] at top level,
so mdx execution failed with [Sys_error: No such file or directory].
Wrap as [let report path = ...] so the snippet typechecks without
hitting the filesystem, and switch [Printf.printf] to [Fmt.pr] for
consistency.