Fix codecov badge in readme

Author: coretl

template/README.md.jinja

@@ -1,5 +1,5 @@
 [![CI]({{repo_url}}/actions/workflows/ci.yml/badge.svg)]({{repo_url}}/actions/workflows/ci.yml)
-[![Coverage](https://codecov.io/gh/{{github_org}}/{{repo_name}}/branch/master/graph/badge.svg)](https://codecov.io/gh/{{github_org}}/{{repo_name}})
+[![Coverage](https://codecov.io/gh/{{github_org}}/{{repo_name}}/branch/main/graph/badge.svg)](https://codecov.io/gh/{{github_org}}/{{repo_name}})
 [![PyPI](https://img.shields.io/pypi/v/{{pypi_name}}.svg)](https://pypi.org/project/{{pypi_name}})
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)