[Bug Report] In a particular case, there are three point on the right side of table-column's type is selection

### Element UI version
2.13.0

### OS/Browsers version
window10/Chrome 83.0.4103.61（正式版本） （64 位）

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/burton_Cui/pen/BaoEyGN?editable=true%3Dhttps%3A%2F%2Felement.eleme.cn%2F2.13%2F

### Steps to reproduce
screen resolution: 1980*1080
when width attribute was sit to 40, there will be three points

https://element.eleme.cn/2.13/#/zh-CN/component/table you can change the width property to 40 in the multi-line online demo

### What is Expected?
only just checkbox

### What is actually happening?
there are three point on the right side of the checkbox