readme: link to godoc for current main branch (#886)

The GoDoc link currently refers to the initial version of this package, but usually folks are looking
for whatever matches the current main branch. This should be updated whenever the package
version changes.

(cherry picked from commit c598ea5)

Author: dylrich

README.md

@@ -1,6 +1,6 @@
 git2go
 ======
-[![GoDoc](https://godoc.org/github.com/libgit2/git2go?status.svg)](http://godoc.org/github.com/libgit2/git2go) [![Build Status](https://travis-ci.org/libgit2/git2go.svg?branch=main)](https://travis-ci.org/libgit2/git2go)
+[![GoDoc](https://godoc.org/github.com/libgit2/git2go?status.svg)](http://godoc.org/github.com/libgit2/git2go/v33) [![Build Status](https://travis-ci.org/libgit2/git2go.svg?branch=main)](https://travis-ci.org/libgit2/git2go)
 
 Go bindings for [libgit2](http://libgit2.github.com/).