ocaml-retry: enable MDX on lib/retry.mli, fix broken doc example
Run mdx on lib/retry.mli so the {[ ... ]} odoc block now type-checks.
The example referenced free `clock` and `connect_to_server`, used
`Eio.Net.Connection_refused` (which is an Eio.Net.error variant, not
an exception, and the expected predicate type is `exn -> bool`), and
`let ... in` chains that do not parse at the toplevel.
Wrapped in `let run connect_to_server = Eio_main.run @@ ...` so the
example takes the user-supplied operation as a parameter; switched
the predicate to `Failure _` (a real exn) so the should_retry has
the right type.