[bug report] pagination slot do not update bug with variable

### Element UI version
2.13.0

### OS/Browsers version
MAC OSmac / chrome 79.0.3945.88

### Vue version
3.0.0-alpha.1

### Reproduction Link
codepen

### Steps to reproduce

{{pageNo}}/{{Math.ceil(total/pageSize)}}

export default {
data() {
return {
pageNo: 1,
pageSize: 4,
total: 17,
a: 1,
};
},
methods: {
handleCurrentChange(pageNo) {
this.pageNo = pageNo
console.log(this.pageNo, this.total)
this.a
}
}
};

### What is Expected?
请尽快处理或给出处理方案

### What is actually happening?
重现链接暂时没有，请本地测试