Update README.md

Author: swhitty

README.md

@@ -1,5 +1,5 @@
 [![Build](https://github.com/swhitty/KeyValueCoder/actions/workflows/build.yml/badge.svg)](https://github.com/swhitty/KeyValueCoder/actions/workflows/build.yml)
-[![CodeCov](https://codecov.io/gh/swhitty/KeyValueCoder/branch/main/graphs/badge.svg)](https://codecov.io/gh/swhitty/KeyValueCoder/branch/master)
+[![CodeCov](https://codecov.io/gh/swhitty/KeyValueCoder/branch/main/graphs/badge.svg)](https://codecov.io/gh/swhitty/KeyValueCoder/branch/main)
 [![Swift 5.8](https://img.shields.io/badge/swift-5.7%20–%205.8-red.svg?style=flat)](https://developer.apple.com/swift)
 [![License](https://img.shields.io/badge/license-MIT-lightgrey.svg)](https://opensource.org/licenses/MIT)
 [![Twitter](https://img.shields.io/badge/twitter-@simonwhitty-blue.svg)](http://twitter.com/simonwhitty)