CI: Link to test coverage badge for main branch (#74)

Author: brawer

README.md

@@ -1,6 +1,7 @@
 # Diffed Places Pipeline
 
-[![Coverage Status](https://coveralls.io/repos/github/diffed-places/pipeline/badge.svg)](https://coveralls.io/github/diffed-places/pipeline)
+[![Coverage Status](https://coveralls.io/repos/github/diffed-places/pipeline/badge.svg?branch=main)](https://coveralls.io/github/diffed-places/pipeline?branch=main)
+
 [![OpenSSF Scorecard](https://api.scorecard.dev/projects/github.com/diffed-places/diffed-places-pipeline/badge)](https://scorecard.dev/viewer/?uri=github.com/diffed-places/diffed-places-pipeline)
 
 Pipeline for the Diffed Places Project.