Fix coverage status

README.md

@@ -2,7 +2,7 @@
 
 [![GitHub Release](https://img.shields.io/github/release/emqx/emqx?color=brightgreen)](https://github.com/emqx/emqx/releases)
 [![Build Status](https://travis-ci.org/emqx/emqx.svg)](https://travis-ci.org/emqx/emqx)
-[![Coverage Status](https://coveralls.io/repos/github/emqx/emqx/badge.svg)](https://coveralls.io/github/emqx/emqx)
+[![Coverage Status](https://coveralls.io/repos/github/emqx/emqx/badge.svg?branch=master)](https://coveralls.io/github/emqx/emqx?branch=master)
 [![Docker Pulls](https://img.shields.io/docker/pulls/emqx/emqx)](https://hub.docker.com/r/emqx/emqx)
 [![Slack Invite](<https://slack-invite.emqx.io/badge.svg>)](https://slack-invite.emqx.io)
 [![Twitter](https://img.shields.io/badge/Twitter-EMQ%20X-1DA1F2?logo=twitter)](https://twitter.com/emqtt)