doc: add classes to version tags in source install instructions

This will allow us to mechanically substitute these strings
using javascript (in a forthcoming change to x/tools/godoc).

Updates #14371

Change-Id: I96e876283060ffbc9f3eabaf55d6b880685453e1
Reviewed-on: https://go-review.googlesource.com/22055


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/22069