Lwt_io.default_buffer_size: fix grammar error
Lwt_io.default_buffer_size: fix grammar error

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