docs(readme): update ci badge syntax (#749)

Author: Fdawgs

README.md

@@ -1,6 +1,6 @@
 # fast-json-stringify
 
-![CI](https://github.com/fastify/fast-json-stringify/workflows/CI/badge.svg)
+[![CI](https://github.com/fastify/fast-json-stringify/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/fastify/fast-json-stringify/actions/workflows/ci.yml)
 [![NPM version](https://img.shields.io/npm/v/fast-json-stringify.svg?style=flat)](https://www.npmjs.com/package/fast-json-stringify)
 [![js-standard-style](https://img.shields.io/badge/code%20style-standard-brightgreen.svg?style=flat)](https://standardjs.com/)
 [![NPM downloads](https://img.shields.io/npm/dm/fast-json-stringify.svg?style=flat)](https://www.npmjs.com/package/fast-json-stringify)