go/version: use "custom" as an example of a version suffix
The suffix in a non-standard toolchain version can be any string. Show
more of a middle ground example of a non-standard version suffix,
aligning it with the example used at https://go.dev/doc/toolchain#name.
For #75953.
Change-Id: I98f9c4de98316aecf76c017eb13cf25582c8939c
Reviewed-on: https://go-review.googlesource.com/c/go/+/720621 Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Mark Freeman <markfreeman@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>