解决uniapp折叠面板报错this.collapse.onChange is not a function

实测uni-collapse-item组件需要用uni-collapse包裹，否则就会报此错误

<uni-collapse>
   <uni-collapse-item title="默认开启" :open="true">
     <text>折叠内容</text>
   </uni-collapse-item>
 </uni-collapse>