[Bug Report] element文档-Progress 进度条-缺少一个参数

### Element UI version
2.13.2

### OS/Browsers version
无

### Vue version
3.0.0-rc.10

### Reproduction Link
https://codepen.io/pen

### Steps to reproduce

export default {
methods: {
format(percentage) {
return percentage === 100 ? '满' : `${percentage}%`;
}
}
};

### What is Expected?
format可以理解为自定义文字,但是在文档里没有,希望加上

### What is actually happening?
没有