replaced main with master in pipeline badge

README.md

@@ -6,7 +6,7 @@ SPDX-License-Identifier: BSD-3-Clause
 
 # PET
 
-[![CI](https://gitlab.dkrz.de/ch1187/plugins4freva/pet/badges/main/pipeline.svg)](https://gitlab.dkrz.de/ch1187/plugins4freva/pet/-/pipelines?page=1&scope=all&ref=main)
+[![CI](https://gitlab.dkrz.de/ch1187/plugins4freva/pet/badges/master/pipeline.svg)](https://gitlab.dkrz.de/ch1187/plugins4freva/pet/-/pipelines?page=1&scope=all&ref=master)
 <!-- [![Code coverage](https://gitlab.dkrz.de/ch1187/plugins4freva/pet/badges/main/coverage.svg)](https://gitlab.dkrz.de/ch1187/plugins4freva/pet/-/graphs/main/charts) -->
 <!-- TODO: uncomment the following line when the package is registered at https://readthedocs.org -->
 <!-- [![Docs](https://readthedocs.org/projects/pet/badge/?version=latest)](https://pet.readthedocs.io/en/latest/) -->