net: min_pmtu default is 552

Small fix in Documentation, since min_pmtu is 512 + 20 + 20 = 552
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -20,7 +20,7 @@ ip_no_pmtu_disc - BOOLEAN
 	default FALSE
 
 min_pmtu - INTEGER
-	default 562 - minimum discovered Path MTU
+	default 552 - minimum discovered Path MTU
 
 route/max_size - INTEGER
 	Maximum number of routes allowed in the kernel.  Increase