Documentation/sysctl/net.txt: fix a typo

s/spicified/specified

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>