btrfs-progs: correct prompt of minimal num of devs for raid56

For btrfs,
Raid5 can't go below 2 devs, not 3;
Raid6 can't go below 3 devs, not 4.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
1 file changed