Documentation: fix wrong value in md.txt

In the current Documentation/md.txt, the lower limit value of
stripe_cache_size is 16 and the default value is 128, but when
I update kernel to the latest mainline version and RAID5 array
is created by mdadm, then execute the following commands, it
shows an error and a difference respectively.

1) set stripe_cache_size to 16
[root@localhost ~]# echo 16 > /sys/block/md0/md/stripe_cache_size
bash: echo: write error: Invalid argument
2) read the default value of stripe_cache_size
[root@localhost ~]# cat /sys/block/md0/md/stripe_cache_size
256

I read drivers/md/raid5.c and find the following related code:
1) in function 'raid5_set_cache_size':
if (size <= 16 || size > 32768)
	return -EINVAL;
2) #define NR_STRIPES		256

So the lower limit value of stripe_cache_size should be 17 and
the default value should be 256.

Signed-off-by: Tiezhu Yang <kernelpatch@126.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>