There are too many contents in the third level of the [bug report] folding menu, which requires scrollbars to appear on the whole page

### Element UI version
2.13.2

### OS/Browsers version
windows 10

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/pen/?&editable=true=https%3A%2F%2Felement.eleme.cn%2F

### Steps to reproduce
1，第二级菜单添加7,8个字菜单
2，折叠点击1级菜单，然后再点2级菜单，浏览器里面出现滚动条

### What is Expected?
不顶到底部出现滚动条

### What is actually happening?
出现滚动条了