docs: smb.conf: fix max read/write/trans default values (bug #9871)

Commit 6d128aac119d948f0ecb0dcf6b400b4eb4027fe6 has increased the limit:
"s3:smb2_server increase defaults for read/write/trans sizes to 1MB"

Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri May 10 23:14:50 CEST 2013 on sn-devel-104