docs: Update GitHub Actions status badge target URLs (#551)

* Update the URLs for the GitHub Actions status badge to show
  the state of the last run on the CI on the default branch, which in
  this case is 'master'.

Author: matthewfeickert

README.md

@@ -1,7 +1,7 @@
 # ServiceX Client Library
 
 
-[![GitHub Actions Status](https://github.com/ssl-hep/ServiceX_frontend/workflows/CI/CD/badge.svg?branch=master)](https://github.com/ssl-hep/ServiceX_frontend/actions)
+[![GitHub Actions Status](https://github.com/ssl-hep/ServiceX_frontend/actions/workflows/ci.yaml/badge.svg)](https://github.com/ssl-hep/ServiceX_frontend/actions/workflows/ci.yaml?query=branch%3Amaster)
 [![Code Coverage](https://codecov.io/gh/ssl-hep/ServiceX_frontend/graph/badge.svg)](https://codecov.io/gh/ssl-hep/ServiceX_frontend)
 
 [![PyPI version](https://badge.fury.io/py/servicex.svg)](https://badge.fury.io/py/servicex)