[release-branch.go1.7] doc: update version tag in source install instructions
[release-branch.go1.7] doc: update version tag in source install instructions

Change-Id: Id83e0371b7232b01be83640ef1e47f9026cf2a23
Reviewed-on: https://go-review.googlesource.com/27055
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-on: https://go-review.googlesource.com/27056
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>

4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go