Update README.md

specify branch for test status badge

README.md

@@ -2,7 +2,7 @@
 
 <img src=https://raw.githubusercontent.com/Dana-Farber-AIOS/pathml/master/docs/source/_static/images/overview.png width="750">
 
-![tests](https://github.com/Dana-Farber-AIOS/pathml/actions/workflows/tests-conda.yml/badge.svg?branch=dev)
+![tests](https://github.com/Dana-Farber-AIOS/pathml/actions/workflows/tests-conda.yml/badge.svg?branch=master)
 [![Documentation Status](https://readthedocs.org/projects/pathml/badge/?version=latest)](https://pathml.readthedocs.io/en/latest/?badge=latest)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 [![PyPI version](https://img.shields.io/pypi/v/pathml)](https://pypi.org/project/pathml/)