doc: change git clone url

doc: change git clone url

The git clone url should be a Gerrit host and not Github, otherwise the
codereview command will fail.

	git-codereview: failed to load Gerrit origin: git origin must be a Gerrit host, not GitHub: https://github.com/golang/go

Change-Id: I62f62c86ee6dce0720a844fc56340135dfae8405
Reviewed-on: https://go-review.googlesource.com/117178
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>