Mapping examples between libgit2 and git2go in README need to be updated

[otherpaco]

In this list:

https://github.com/libgit2/git2go#which-go-version-to-use

One should use libgit2 v1.5 with git2go v34

The text implies libgit2 v1.2 with git2go v34

"You can import them in your project with the version's major number as a suffix. For example, if you have libgit2 v1.2 installed, you'd import git2go v34 with:"

I updated the two relevant sentences in this PR #939