message: fix mask type
diff: @@ -109,7 +109,7 @@ export default class DisplayObject extends EventEmitter
  /**
  * The original, cached mask of the object
  *
  - * @member {PIXI.Rectangle}
  + * @member {PIXI.Graphics|PIXI.Sprite}
  * @private
  */
  this._mask = null;
  