Lwt_unix: fix typo
Lwt_unix: fix typo

[skip ci]

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