Update coverage badge link from 'master' to 'main'

Author: thrau

README.md

@@ -4,7 +4,7 @@ Verdin
 <p>
   <a href="https://pypi.org/project/verdin/"><img alt="PyPI Version" src="https://img.shields.io/pypi/v/verdin?color=blue"></a>
   <a href="https://github.com/localstack/verdin/actions/workflows/build.yml"><img alt="CI Status" src="https://github.com/localstack/verdin/actions/workflows/build.yml/badge.svg"></a>
-  <a href="https://coveralls.io/github/localstack/verdin?branch=master"><img src="https://coveralls.io/repos/github/localstack/verdin/badge.svg?branch=master" alt="Coverage Status" /></a>
+  <a href="https://coveralls.io/github/localstack/verdin?branch=main"><img src="https://coveralls.io/repos/github/localstack/verdin/badge.svg?branch=main" alt="Coverage Status" /></a>
   <a href="https://img.shields.io/pypi/l/verdin.svg"><img alt="PyPI License" src="https://img.shields.io/pypi/l/verdin.svg"></a>
   <a href="https://github.com/psf/black"><img alt="Code style: black" src="https://img.shields.io/badge/code%20style-black-000000.svg"></a>
 </p>