readme: Fix codecov badge URL

Author: bep

README.md

@@ -1,6 +1,6 @@
 [![Tests on Linux, MacOS and Windows](https://github.com/bep/imagemeta/workflows/Test/badge.svg)](https://github.com/bep/imagemeta/actions?query=workflow:Test)
 [![Go Report Card](https://goreportcard.com/badge/github.com/bep/imagemeta)](https://goreportcard.com/report/github.com/bep/imagemeta)
-[![codecov](https://codecov.io/gh/bep/imagemeta/branch/master/graph/badge.svg)](https://codecov.io/gh/bep/imagemeta)
+[![codecov](https://codecov.io/gh/bep/imagemeta/branch/main/graph/badge.svg)](https://codecov.io/gh/bep/imagemeta)
 [![GoDoc](https://godoc.org/github.com/bep/imagemeta?status.svg)](https://godoc.org/github.com/bep/imagemeta)
 
 ## This is about READING image metadata