ocaml-osv: enable MDX on lib/osv.mli
Run mdx on lib/osv.mli so the {[ ... ]} odoc block now type-checks.
The example used `env#net` / `env#clock` directly (object-method
shorthand that requires being inside the right scope) and treated
`Osv.query_purl`'s `(_, string) result` return as a `vulnerability
list`. Wrapped in `let run () = Eio_main.run @@ ...`, switched to
`Eio.Stdenv.net` / `Eio.Stdenv.clock`, pattern-matched the result on
both branches with `Fmt.pr` / `Fmt.epr`, and annotated the iter
parameter as `(v : Osv.vulnerability)` so the field accesses resolve
unambiguously.