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

Documentation/networking/ip-sysctl.txt

@@ -594,7 +594,7 @@ tcp_fastopen - INTEGER
 
 tcp_syn_retries - INTEGER
 	Number of times initial SYNs for an active TCP connection attempt
-	will be retransmitted. Should not be higher than 255. Default value
+	will be retransmitted. Should not be higher than 127. Default value
 	is 6, which corresponds to 63seconds till the last retransmission
 	with the current initial RTO of 1second. With this the final timeout
 	for an active TCP connection attempt will happen after 127seconds.