readme: fix coveralls badge

Author: pires

README.md

@@ -1,7 +1,7 @@
 # go-proxyproto
 
 [![Actions Status](https://github.com/pires/go-proxyproto/workflows/test/badge.svg)](https://github.com/pires/go-proxyproto/actions)
-[![Coverage Status](https://coveralls.io/repos/github/pires/go-proxyproto/badge.svg?branch=master)](https://coveralls.io/github/pires/go-proxyproto?branch=master)
+[![Coverage Status](https://coveralls.io/repos/github/pires/go-proxyproto/badge.svg?branch=main)](https://coveralls.io/github/pires/go-proxyproto?branch=main)
 [![Go Report Card](https://goreportcard.com/badge/github.com/pires/go-proxyproto)](https://goreportcard.com/report/github.com/pires/go-proxyproto)
 [![](https://godoc.org/github.com/pires/go-proxyproto?status.svg)](https://pkg.go.dev/github.com/pires/go-proxyproto?tab=doc)