Documentation: fix neigh/default/gc_thresh1 default value.

The default value is 128, not 256
	#grep gc_thresh1 net/ -rI
	net/decnet/dn_neigh.c:	.gc_thresh1 =			128,
	net/ipv6/ndisc.c:	.gc_thresh1 =	 128,
	net/ipv4/arp.c:	.gc_thresh1	= 128,

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -29,7 +29,7 @@ route/max_size - INTEGER
 neigh/default/gc_thresh1 - INTEGER
 	Minimum number of entries to keep.  Garbage collector will not
 	purge entries if there are fewer than this number.
-	Default: 256
+	Default: 128
 
 neigh/default/gc_thresh3 - INTEGER
 	Maximum number of neighbor entries allowed.  Increase this