sctp: fix up a spacing

fix up spacing of proc_sctp_do_hmac_alg for according to the
proc_sctp_do_rto_min[max] in sysctl.c

Suggested-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Wang Weidong <wangweidong1@huawei.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>