message: chore: update downloads badge to point to graph of downloads
diff: <p align="center">
  <a href="https://circleci.com/gh/vuejs/vue/tree/dev"><img src="https://img.shields.io/circleci/project/vuejs/vue/dev.svg" alt="Build Status"></a>
  <a href="https://codecov.io/github/vuejs/vue?branch=dev"><img src="https://img.shields.io/codecov/c/github/vuejs/vue/dev.svg" alt="Coverage Status"></a>
  - <a href="https://www.npmjs.com/package/vue"><img src="https://img.shields.io/npm/dm/vue.svg" alt="Downloads"></a>
  + <a href="https://npmcharts.com/compare/vue?minimal=true"><img src="https://img.shields.io/npm/dm/vue.svg" alt="Downloads"></a>
  <a href="https://www.npmjs.com/package/vue"><img src="https://img.shields.io/npm/v/vue.svg" alt="Version"></a>
  <a href="https://www.npmjs.com/package/vue"><img src="https://img.shields.io/npm/l/vue.svg" alt="License"></a>
  <a href="https://chat.vuejs.org/"><img src="https://img.shields.io/badge/chat-on%20discord-7289da.svg" alt="Chat"></a>
  