Update README.md

Author: jennyfothergill

README.md

@@ -1,7 +1,7 @@
 # GRiTS : Grits Reduces/Restores Topology with SMARTS/SMILES
 [![build](https://github.com/cmelab/grits/actions/workflows/build_cpu.yml/badge.svg)](https://github.com/cmelab/grits/actions/workflows/build_cpu.yml)
 [![pytest](https://github.com/cmelab/grits/actions/workflows/pytest.yml/badge.svg)](https://github.com/cmelab/grits/actions/workflows/pytest.yml)
-[![codecov](https://codecov.io/gh/cmelab/grits/branch/main/graph/badge.svg?token=lGG8Zf65HP)](https://codecov.io/gh/cmelab/grits)
+[![codecov](https://codecov.io/gh/cmelab/grits/branch/master/graph/badge.svg?token=lGG8Zf65HP)](https://codecov.io/gh/cmelab/grits)
 
 ![GRiTS workflow](/.github/grits.png)