Fix the codecov.io shield in README

The branch name should be main not master.

Author: aucampia

README.md

@@ -5,7 +5,7 @@
 
 
 [![Go Report Card](https://goreportcard.com/badge/github.com/coopnorge/go-masker-lib)](https://goreportcard.com/report/github.com/coopnorge/go-masker-lib)
-[![codecov](https://codecov.io/gh/coopnorge/go-masker-lib/branch/master/graph/badge.svg)](https://codecov.io/gh/coopnorge/go-masker-lib)
+[![codecov](https://codecov.io/gh/coopnorge/go-masker-lib/branch/main/graph/badge.svg)](https://codecov.io/gh/coopnorge/go-masker-lib)
 [![Godoc](https://img.shields.io/badge/godoc-reference-blue.svg)](https://godoc.org/github.com/coopnorge/go-masker-lib)
 [![license](https://img.shields.io/badge/license-MIT-green)](./LICENSE)