README.md: add badge to pkg.go.dev

Change-Id: I90a3334507f4501ee082afeb878b82f71b3392ae
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/275303
Trust: Julie Qiu <[email protected]>
Reviewed-by: Andrew Gerrand <[email protected]>

Author: julieqiu

README.md

@@ -1,7 +1,7 @@
 # OAuth2 for Go
 
 [![Build Status](https://travis-ci.org/golang/oauth2.svg?branch=master)](https://travis-ci.org/golang/oauth2)
-[![GoDoc](https://godoc.org/golang.org/x/oauth2?status.svg)](https://godoc.org/golang.org/x/oauth2)
+[![Go Package Documentation](https://pkg.go.dev/badge/golang.org/x/oauth2.svg)](https://pkg.go.dev/golang.org/x/oauth2)
 
 oauth2 package contains a client implementation for OAuth 2.0 spec.