[release-branch.go1.12] doc/go1.12: change go install to go get

Using go get prevents the failure case of when the
user doesn't have the repo on their machine.

Change-Id: I9c1174087728b5b06b578b0d52df6eeb7e8c7a3c
Reviewed-on: https://go-review.googlesource.com/c/163718


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 2f9728aa)
Reviewed-on: https://go-review.googlesource.com/c/163719


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>