aio_read.2: correct the description of aio_buf

Looks like a copypasta from aio_write.2.

[skip ci]

Reported by: Paul Floyd <pjfloyd@wanadoo.fr>
MFC after: 1 week
Sponsored by: Axcient
Reviewed by: jilles (manpages)
Differential Revision: https://reviews.freebsd.org/D42621