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>