replace event.target to event.currentTarget

[DragorWW]

fix flow error in Select.js, related to #3350

The reason why you want to use event.currentTarget is that event.target may be the wrong element due to event propagation.
see more in:

• Make React.SyntheticEvent.target generic over T, not React.SyntheticEvent.currentTarget DefinitelyTyped/DefinitelyTyped#11508 (comment)
• https://flow.org/en/docs/react/events/

flow types from react

declare class SyntheticEvent<+T: EventTarget = EventTarget, +E: Event = Event> {
  bubbles: boolean;
  cancelable: boolean;
  +currentTarget: T;
  defaultPrevented: boolean;
  eventPhase: number;
  isDefaultPrevented(): boolean;
  isPropagationStopped(): boolean;
  isTrusted: boolean;
  nativeEvent: E;
  preventDefault(): void;
  stopPropagation(): void;
  // This should not be `T`. Use `currentTarget` instead. See:
  // https://github.com/DefinitelyTyped/DefinitelyTyped/issues/11508#issuecomment-256045682
  +target: EventTarget;
  timeStamp: number;
  type: string;
  persist(): void;
}

[gwyneplaine]

lgtm thanks @DragorWW