Lwt_io.block: fix grammar errors
Lwt_io.block: fix grammar errors

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