docs: link to v3 API reference in README (#25)

The "Go Reference" badge in the README
links to the API reference for v1 of the library.
This points it to the v3 documentation.

While at it, this also switches to the official
[pkg.go.dev badge](https://pkg.go.dev/badge/).

Author: abhinav

README.md

@@ -1,6 +1,6 @@
 # LaunchDarkly Streaming JSON for Go
 
-[![Circle CI](https://circleci.com/gh/launchdarkly/go-jsonstream.svg?style=shield)](https://circleci.com/gh/launchdarkly/go-jsonstream) [![Documentation](https://img.shields.io/static/v1?label=go.dev&message=reference&color=00add8)](https://pkg.go.dev/gopkg.in/launchdarkly/go-jsonstream.v1)
+[![Circle CI](https://circleci.com/gh/launchdarkly/go-jsonstream.svg?style=shield)](https://circleci.com/gh/launchdarkly/go-jsonstream) [![Go Reference](https://pkg.go.dev/badge/github.com/launchdarkly/go-jsonstream/v3.svg)](https://pkg.go.dev/github.com/launchdarkly/go-jsonstream/v3)
 
 ## Overview