ggsn: Fix config file writing of IPv6 DNS settings

There was a copy+paste mistake that created syntax errors during the
write of a config file that contained IPv6 DNS server settings.

Change-Id: Ida40c32c72dba8155f8294b93484e46e8bd27739
diff --git a/ggsn/ggsn_vty.c b/ggsn/ggsn_vty.c
index d7bd9a7..d9bbe3e 100644
--- a/ggsn/ggsn_vty.c
+++ b/ggsn/ggsn_vty.c
@@ -642,7 +642,7 @@
 	for (i = 0; i < ARRAY_SIZE(apn->v6.cfg.dns); i++) {
 		if (!apn->v6.cfg.dns[i].len)
 			continue;
-		vty_out(vty, "  ip dns %u %s%s", i, in46a_ntoa(&apn->v6.cfg.dns[i]), VTY_NEWLINE);
+		vty_out(vty, "  ipv6 dns %u %s%s", i, in46a_ntoa(&apn->v6.cfg.dns[i]), VTY_NEWLINE);
 	}
 	if (apn->v6.cfg.ifconfig_prefix.addr.len)
 		vty_dump_prefix(vty, "  ipv6 ifconfig ", &apn->v6.cfg.ifconfig_prefix);