build badge for main branch only

Author: pvsr

README.md

@@ -1,6 +1,6 @@
 # qutebrowser profile manager
 
-[![builds.sr.ht status](https://builds.sr.ht/~pvsr/qbpm.svg)](https://builds.sr.ht/~pvsr/qbpm?)
+[![builds.sr.ht status](https://builds.sr.ht/~pvsr/qbpm/commits/main.svg)](https://builds.sr.ht/~pvsr/qbpm/commits/main?)
 
 qutebrowser profile manager (qbpm) is a tool for creating and managing
 [qutebrowser](https://github.com/qutebrowser/qutebrowser) profiles. There isn't