docs: updated godoc badge (lni#251)

README.md

@@ -4,7 +4,7 @@
 ![Build status](https://github.com/lni/dragonboat/workflows/Test/badge.svg?branch=master)
 [![Go Report Card](https://goreportcard.com/badge/github.com/lni/dragonboat)](https://goreportcard.com/report/github.com/lni/dragonboat)
 [![codecov](https://codecov.io/gh/lni/dragonboat/branch/master/graph/badge.svg)](https://codecov.io/gh/lni/dragonboat)
-[![Godoc](http://img.shields.io/badge/go-documentation-blue.svg)](https://godoc.org/github.com/lni/dragonboat)
+[![Godoc](http://img.shields.io/badge/go-documentation-blue.svg)](https://godoc.org/github.com/lni/dragonboat/v3)
 [![Join the chat at https://gitter.im/lni/dragonboat](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/lni/dragonboat?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 ## News ##