[release-branch.go1] doc: clarify that the displayed tar file name is an example

««« CL 6573065 / 49b27543b08b
doc: clarify that the displayed tar file name is an example

R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/6573065
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/6571068