How can I put a callout in an annotation of the mapview swift 4

I'm trying to show a callout (a small box with the title) in the pins that I show in the mapview but trying to do it directly shows me the title below the pin shown on the map.

for i in self.listaInstructor {
    print("CICLO DE INSTRUCTOR")
    let latitud = Double("\((i as! NSDictionary).value(forKey: "latitud")!)")!
    let longitud = Double("\((i as! NSDictionary).value(forKey: "longitud")!)")!
    let nombre = String("\((i as! NSDictionary).value(forKey: "nombre")!)")
    let apellido = String("\((i as! NSDictionary).value(forKey: "apellido")!)")

    let annotation = MKPointAnnotation()
    annotation.coordinate = CLLocationCoordinate2D(latitude: latitud, longitude: longitud)
    annotation.title = "\(nombre) \(apellido)"

    self.mapView.addAnnotation(annotation)
}

and I'm using the following function to show the callout

func mapView(_ mapView: MKMapView, ViewFor annotation: MKAnnotation) -> MKAnnotationView? {

    if annotation is  MKUserLocation{
        return nil
    }

    let pin = MKPinAnnotationView(annotation: annotation, reuseIdentifier: "pin")
    pin.canShowCallout = true

    return pin
}

and the only thing I get from the result is the following