Show drone status for master branch in README (#12248)

README.md

@@ -2,7 +2,7 @@
 
 <h1> <img src="https://raw.githubusercontent.com/go-gitea/gitea/master/public/img/gitea-192.png" alt="logo" width="30" height="30"> Gitea - Git with a cup of tea</h1>
 
-[![Build Status](https://drone.gitea.io/api/badges/go-gitea/gitea/status.svg)](https://drone.gitea.io/go-gitea/gitea)
+[![Build Status](https://drone.gitea.io/api/badges/go-gitea/gitea/status.svg?ref=refs/heads/master)](https://drone.gitea.io/go-gitea/gitea)
 [![Join the Discord chat at https://discord.gg/Gitea](https://img.shields.io/discord/322538954119184384.svg)](https://discord.gg/Gitea)
 [![](https://images.microbadger.com/badges/image/gitea/gitea.svg)](https://microbadger.com/images/gitea/gitea "Get your own image badge on microbadger.com")
 [![codecov](https://codecov.io/gh/go-gitea/gitea/branch/master/graph/badge.svg)](https://codecov.io/gh/go-gitea/gitea)