tcp: Correction to RFC number in comment

Challenge ACK is described in RFC 5961, fix typo.
Signed-off-by: NSowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -5029,7 +5029,7 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	/* step 3: check security and precedence [ignored] */
 
 	/* step 4: Check for a SYN
-	 * RFC 5691 4.2 : Send a challenge ack
+	 * RFC 5961 4.2 : Send a challenge ack
 	 */
 	if (th->syn) {
 syn_challenge: