net: change tcp_syn_retries documentation

Documentation should be kept consistent with the code:

 static int tcp_syn_retries_max = MAX_TCP_SYNCNT;
 #define MAX_TCP_SYNCNT          127

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>