element Table modify the header style and add prompts

​​​​​​​Just set slot="header" in the template

    <el-table-column prop="name">
      <template slot="header" slot-scope="scope">
        <span>名字</span>
        <el-tooltip class="item" effect="dark" content="这是提示文字" placement="right">
          <i class="el-icon-question"></i>
        </el-tooltip>
      </template>
    </el-table-column>

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