Lwt_io.open_file docs: use OCaml octal notation
Lwt_io.open_file docs: use OCaml octal notation

See https://github.com/ocsigen/lwt/issues/587#issuecomment-386737784.

[skip ci]

rneswold/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt,ocsigen/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt,c-cube/lwt