readme: restrict CI badge to master branch

Author: emersion

README.md

@@ -1,7 +1,7 @@
 # go-message
 
 [![godocs.io](https://godocs.io/github.com/emersion/go-message?status.svg)](https://godocs.io/github.com/emersion/go-message)
-[![builds.sr.ht status](https://builds.sr.ht/~emersion/go-message/commits.svg)](https://builds.sr.ht/~emersion/go-message/commits?)
+[![builds.sr.ht status](https://builds.sr.ht/~emersion/go-message/commits/master.svg)](https://builds.sr.ht/~emersion/go-message/commits/master?)
 
 A Go library for the Internet Message Format. It implements: