每页序号从1开始：
<el-table-column label="序号" type="index" width="50" align="center" />序号跟随翻页：
<el-table-column label="序号" type="index" width="50" align="center"><template slot-scope="scope"><span>{
{ (pageNum - 1) * pageSize + scope.$index + 1 }}</span></template>
</el-table-column>