fix: fixing branch name for code cov badge

Signed-off-by: Travis Payne <travis.payne@accenture.com>

README.md

@@ -2,7 +2,7 @@
 
 # Hyperledger Cactus
 
- ![license](https://img.shields.io/github/license/hyperledger/cactus) [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/4089/badge)](https://bestpractices.coreinfrastructure.org/projects/4089) [![codecov](https://codecov.io/gh/hyperledger/cactus/branch/master/graph/badge.svg?token=BJklIsqf1S)](https://codecov.io/gh/hyperledger/cactus)
+ ![license](https://img.shields.io/github/license/hyperledger/cactus) [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/4089/badge)](https://bestpractices.coreinfrastructure.org/projects/4089) [![codecov](https://codecov.io/gh/hyperledger/cactus/branch/main/graph/badge.svg?token=BJklIsqf1S)](https://codecov.io/gh/hyperledger/cactus)
  ![GitHub issues](https://img.shields.io/github/issues/hyperledger/cactus) ![Travis (.org)](https://img.shields.io/travis/hyperledger/cactus)
 
 This project is an _Incubation_ Hyperledger project. For more information on the history of this project see the [Cactus wiki page](https://wiki.hyperledger.org/display/cactus). Information on what _Active_ entails can be found in