net: fix AF_SMC related typo

When introducing the new socket family AF_SMC in
commit ac713874 ("smc: establish new socket family"),
a typo in af_family_clock_key_strings has slipped in.
This patch repairs it.

Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Fixes: ac713874 ("smc: establish new socket family")
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>