tcp: doc : fix the syncookies default value

syncookies is on for default since in commit e994b7c9
(tcp: Don't make syn cookies initial setting depend on CONFIG_SYSCTL).

And fix a typo of CONFIG_SYN_COOKIES.

Signed-off-by: Shan Wei <davidshan@tencent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>