ocaml-cam: enable MDX on lib/cam.mli, fix broken doc example
Run mdx on lib/cam.mli so the {[ ... ]} odoc block now type-checks.
The example used `let cdm = ... in` where `...` is not OCaml syntax.
Wrapped the avoid + Printf.printf flow in
`let plan_burn (cdm : Cdm.t) = ...` so the example takes a real
typed CDM as input and shows both the Some / None branches of the
avoid search.