mdadm/grow: reshape would be stuck from raid1 to raid5

mdadm/grow: reshape would be stuck from raid1 to raid5

systemctl doesn't interpret mdadm-grow-continue@.service
correctly due to the wrong argument provided in [service],
it should be corrected %I as %i. Otherwise, if the service
cannot start by systemctl and the reshap progress would be
stuck all time when grows array from raid1 to raid5.

reproduce steps:
./mdadm -CR /dev/md0 -l1 -b internal -n2 /dev/loop[0-1]
./mdadm --grow /dev/md0 -l5 -n3 -a /dev/loop2

Signed-off-by: Zhilong Liu <zlliu@suse.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@gmail.com>