Make the documentation for `go get` match the documentation for `go
install`, since `go get` essentially invokes `go install`.

Update #15825.

Change-Id: I374d80efd301814b6d98b86b7a4a68dd09704c92
Reviewed-on: https://go-review.googlesource.com/23925Reviewed-by: Andrew Gerrand <adg@golang.org>