adapt CI Badge URL (#128)

Author: JoshuaLampert

README.md

@@ -2,7 +2,7 @@
 
 [![Docs-stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://trixi-framework.github.io/P4est.jl/stable)
 [![Slack](https://img.shields.io/badge/chat-slack-e01e5a)](https://join.slack.com/t/trixi-framework/shared_invite/zt-sgkc6ppw-6OXJqZAD5SPjBYqLd8MU~g)
-[![Build Status](https://github.com/trixi-framework/P4est.jl/workflows/CI/badge.svg)](https://github.com/trixi-framework/P4est.jl/actions?query=workflow%3ACI)
+[![Build Status](https://github.com/trixi-framework/P4est.jl/actions/workflows/ci.yml/badge.svg)](https://github.com/trixi-framework/P4est.jl/actions?query=workflow%3ACI)
 [![Codecov](https://codecov.io/gh/trixi-framework/P4est.jl/branch/main/graph/badge.svg)](https://codecov.io/gh/trixi-framework/P4est.jl)
 [![Coveralls](https://coveralls.io/repos/github/trixi-framework/P4est.jl/badge.svg?branch=main)](https://coveralls.io/github/trixi-framework/P4est.jl?branch=main)
 [![Aqua QA](https://raw.githubusercontent.com/JuliaTesting/Aqua.jl/master/badge.svg)](https://github.com/JuliaTesting/Aqua.jl)