Lwt_io: fix grammar error in several functions
Lwt_io: fix grammar error in several functions

`open` should be `opens`.

rgrinberg/lwt,fdopen/lwt,c-cube/lwt,rgrinberg/lwt,ocsigen/lwt,c-cube/lwt,rgrinberg/lwt,mfp/lwt,rneswold/lwt,rgrinberg/lwt,gfxmonk/lwt,ocsigen/lwt,rneswold/lwt,mirage/lwt,fdopen/lwt,gfxmonk/lwt,ocsigen/lwt,mfp/lwt,c-cube/lwt,mirage/lwt,rneswold/lwt