Apply suggestions from code review

Co-authored-by: Thomas Gran <[email protected]>

application/src/main/java/org/opentripplanner/service/vehicleparking/internal/DefaultVehicleParkingService.java

@@ -52,7 +52,7 @@ public Collection<VehicleParking> listVehicleParkings() {
   }
 
   @Override
-  public ImmutableListMultimap<VehicleParkingGroup, VehicleParking> listVehicleParkingGroups() {
+  public ListMultimap<VehicleParkingGroup, VehicleParking> listVehicleParkingGroups() {
     return repository.getVehicleParkingGroups();
   }