Fix the image URL of the CI status badge in the README

[oliverklee]

Currently, the CI status badge shows “no status”. We need to update the image URL to the new format as documented in https://docs.github.com/en/actions/monitoring-and-troubleshooting-workflows/monitoring-workflows/adding-a-workflow-status-badge.

https://github.com/MyIntervals/PHP-CSS-Parser/pull/694 did this for another project that had the same issue.