md: remove the extra line for ->hot_add_disk

It is not not necessary to add a newline for them since they don't exceed
80 characters, and it is not intutive to distinguish ->hot_add_disk() from
hot_add_disk() too.

Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Song Liu <songliubraving@fb.com>