ocaml-http: fix huri.mli example to compile under MDX
The example used 'let .. in ..' top-level form (only valid inside an
expression), bare 'Uri' / 'Huri' references that don't resolve under
'open Http', and a top-level Eio.Buf_write.with_flow that needed a
'flow' value MDX can't bind. Rewrite as let-bindings, qualify Huri as
Http.Huri, and wrap the with_flow call in a 'run flow' function so the
example is callable rather than a side-effect at module init.