modify question that the bird will fly to out of the screen

[majiajia]

Bird.js _updatePosition
let visibleSize = cc.director.getVisibleSize();
if (this.node.y >= visibleSize.height / 2) {
this.node.y = visibleSize.height / 2 - 10;
}

if the bird is out of screen ,the bird will be just be the top of the screen