message: docs: update badge style
diff: <h3 align="center" style="margin: 30px 0 35px;">Mobile UI Components built on Vue</h3>
  <p align="center">
  - <img src="https://img.shields.io/npm/v/vant.svg?style=flat-square" alt="npm version" />
  - <img src="https://img.shields.io/codecov/c/github/youzan/vant/dev.svg?style=flat-square" alt="Coverage Status" />
  - <img src="https://github.com/youzan/vant/workflows/CI/badge.svg" alt="CI" />
  - <img src="https://img.shields.io/npm/dt/vant.svg?style=flat-square" alt="downloads" />
  - <img src="https://img.shields.io/npm/dm/vant.svg?style=flat-square" alt="downloads" />
  - <img src="https://img.badgesize.io/https://unpkg.com/vant/lib/vant.min.js?compression=gzip&style=flat-square&label=JS%20gzip%20size" alt="JS Gzip Size" />
  - <img src="https://img.badgesize.io/https://unpkg.com/vant/lib/index.css?compression=gzip&style=flat-square&label=CSS%20gzip%20size" alt="CSS Gzip Size" />
  + <img src="https://img.shields.io/npm/v/vant.svg?style=for-the-badge" alt="npm version" />
  + <img src="https://img.shields.io/github/workflow/status/youzan/vant/CI/dev?style=for-the-badge" alt="npm version" />
  + <img src="https://img.shields.io/codecov/c/github/youzan/vant/dev.svg?style=for-the-badge&color=#4fc08d" alt="Coverage Status" />
  + <img src="https://img.shields.io/npm/dm/vant.svg?style=for-the-badge&color=#4fc08d" alt="downloads" />
  + <img src="https://img.badgesize.io/https://unpkg.com/vant/lib/vant.min.js?compression=gzip&style=for-the-badge&label=gzip%20size&color=#4fc08d" alt="Gzip Size" />
  </p>
  <p align="center">
  