docs(readme): update ci badge target (#1034)

Signed-off-by: Frazer Smith <[email protected]>

Author: Fdawgs

README.md

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