Add badge for main branch

Author: tobiasjaster

README.md

@@ -1,7 +1,6 @@
 # go-canopen
 
-[main]: https://github.com/jaster-prj/go-canopen/actions/workflows/ci.yml
-[main-svg]: https://github.com/jaster-prj/go-canopen/actions/workflows/ci.yml/badge.svg
+[![main CI](https://github.com/jaster-prj/go-canopen/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/jaster-prj/go-canopen/actions/workflows/ci.yml)
 
 Implement canopen protocol with [https://github.com/angelodlfrtr/go-can](https://github.com/angelodlfrtr/go-can)