docs(readme): update go reference badge to point to v2 and new domain (#1087)

Author: andreynering

README.md

@@ -2,7 +2,7 @@
 
 # A general purpose syntax highlighter in pure Go
 
-[![Golang Documentation](https://godoc.org/github.com/alecthomas/chroma?status.svg)](https://godoc.org/github.com/alecthomas/chroma) [![CI](https://github.com/alecthomas/chroma/actions/workflows/ci.yml/badge.svg)](https://github.com/alecthomas/chroma/actions/workflows/ci.yml) [![Slack chat](https://img.shields.io/static/v1?logo=slack&style=flat&label=slack&color=green&message=gophers)](https://invite.slack.golangbridge.org/)
+[![Go Reference](https://pkg.go.dev/badge/github.com/alecthomas/chroma/v2.svg)](https://pkg.go.dev/github.com/alecthomas/chroma/v2) [![CI](https://github.com/alecthomas/chroma/actions/workflows/ci.yml/badge.svg)](https://github.com/alecthomas/chroma/actions/workflows/ci.yml) [![Slack chat](https://img.shields.io/static/v1?logo=slack&style=flat&label=slack&color=green&message=gophers)](https://invite.slack.golangbridge.org/)
 
 
 Chroma takes source code and other structured text and converts it into syntax