ocaml-http: fix README quick start against the real API
The block was off by several names: bare [Body] / [Headers] / [Mime]
without [open Http], a positional [Multipart.name] field that
actually lives in [Body.part], a [Mime.pdf] constant that doesn't
exist, [Body.json] called with [Jsont.Object] instead of
[Json.Value.object'], and missing [nox-json] / [eio*] mdx libraries.
Add [open Http], reach into [Json.Value.(...)] to construct the JSON
tree, use [Mime.of_string "application/pdf"], and label the part
record with [Body.name]. Wrap the multipart upload as [let upload
~fs = ...] since it needs an [Eio] FS.