message: docs: update npm shields link
diff: <p align="center">Mobile UI Components built on Vue</p>
  <p align="center">
  - <img src="https://img.shields.io/npm/v/vant/next?style=flat-square" alt="npm version" />
  + <img src="https://img.shields.io/npm/v/vant?style=flat-square" alt="npm version" />
  <img src="https://img.shields.io/github/workflow/status/youzan/vant/CI/dev?style=flat-square" alt="CI Status" />
  <img src="https://img.shields.io/codecov/c/github/youzan/vant/dev.svg?style=flat-square&color=#4fc08d" alt="Coverage Status" />
  <img src="https://img.shields.io/npm/dm/vant.svg?style=flat-square&color=#4fc08d" alt="downloads" />
  