ocaml-respond: defer README example via let run ()
The block called [Eio_main.run] at top level (mdx executes it) and
referenced a non-existent [Respond.post_request_body]; the body is
just [req.body] on the [post_request] record. Wrap as [run () = ...]
and use the right field. Add [eio] / [eio.unix] to the mdx libs.