provider/google: Route doc zone update

Author: Lars Wander

website/source/docs/providers/google/r/compute_route.html.markdown

@@ -45,7 +45,7 @@ The following arguments are supported:
 * `next_hop_instance` - (Optional) The name of the VM instance to route to
     if this route is matched.
 
-* `next_hop_instance_zone` - (Optional) The zone of the instance specified
+* `next_hop_instance_zone` - (Required when `next_hop_instance` is specified) The zone of the instance specified
     in `next_hop_instance`.
 
 * `next_hop_gateway` - (Optional) The name of the internet gateway to route