Update README.md

Author: z4kn4fein

README.md

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 # ConfigCat OpenFeature Provider for Python
 
-[![CI](https://github.com/configcat/openfeature-python/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/configcat/openfeature-python/actions/workflows/ci.yml) 
+[![CI](https://github.com/configcat/openfeature-python/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/configcat/openfeature-python/actions/workflows/ci.yml) 
 [![PyPI](https://img.shields.io/pypi/v/configcat-openfeature-provider.svg)](https://pypi.python.org/pypi/configcat-openfeature-provider)
 [![PyPI](https://img.shields.io/pypi/pyversions/configcat-openfeature-provider.svg)](https://pypi.python.org/pypi/configcat-openfeature-provider)