ocaml-http: enable MDX on lib/huri.mli

Run mdx on lib/huri.mli so the {[ ... ]} odoc block now type-checks.

The example used `let uri = ... in let host = ... in Eio.Buf_write...`
chains and a free `flow` argument. Restructured as toplevel bindings:
real `uri` literal, an `assert (Uri.host uri = Some "example.com")`
documenting the URI-parser's output, and the Buf_write call wrapped
in `let run flow = ...` so the user supplies the Eio sink and the
example doesn't try to write at mdx test time.