Documentation: Fix tcp_challenge_ack_limit default value

The default value of tcp_challenge_ack_limit has been changed from
100 to 1000 and this patch fixes its documentation.
Signed-off-by: Cambda Zhu <cambda@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -812,7 +812,7 @@ tcp_limit_output_bytes - INTEGER
 tcp_challenge_ack_limit - INTEGER
 	Limits number of Challenge ACK sent per second, as recommended
 	in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
-	Default: 100
+	Default: 1000
 
 tcp_rx_skb_cache - BOOLEAN
 	Controls a per TCP socket cache of one skb, that might help