readme: switch back to pkg.go.dev for docs

Author: emersion

README.md

@@ -1,6 +1,6 @@
 # go-smtp
 
-[![godocs.io](https://godocs.io/github.com/emersion/go-smtp?status.svg)](https://godocs.io/github.com/emersion/go-smtp)
+[![Go Reference](https://pkg.go.dev/badge/github.com/emersion/go-smtp.svg)](https://pkg.go.dev/github.com/emersion/go-smtp)
 [![builds.sr.ht status](https://builds.sr.ht/~emersion/go-smtp/commits.svg)](https://builds.sr.ht/~emersion/go-smtp/commits?)
 
 An ESMTP client and server library written in Go.