Corrected router gateway set command

While configuring the external network as the default
router gateway for IPV6 in lib/neutron_plugins/services/l3,
"router" keyword is missing in the command.

Corrected the command.

Change-Id: I055bea5137a841f709d4865ec9a43d6b53f8f4c9
Closes-Bug: 1660712
diff --git a/lib/neutron_plugins/services/l3 b/lib/neutron_plugins/services/l3
index cd0c1ed..00b5a63 100644
--- a/lib/neutron_plugins/services/l3
+++ b/lib/neutron_plugins/services/l3
@@ -385,7 +385,7 @@
     # If the external network has not already been set as the default router
     # gateway when configuring an IPv4 public subnet, do so now
     if [[ "$IP_VERSION" == "6" ]]; then
-        openstack --os-cloud devstack-admin --os-region "$REGION_NAME" set --external-gateway $EXT_NET_ID $ROUTER_ID
+        openstack --os-cloud devstack-admin --os-region "$REGION_NAME" router set --external-gateway $EXT_NET_ID $ROUTER_ID
     fi
 
     # This logic is specific to using the l3-agent for layer 3