cmd/go: keep "go help" in unknown topic suggestion

Fixed missing concatenation to build the suggested command.

Fixes #33153

Change-Id: I26c02f2966247aa07d1447b8ec1a2203046842dd
GitHub-Last-Rev: 9631abf1d0d187cbd94495a2a789e5863b421441
GitHub-Pull-Request: golang/go#33245
Reviewed-on: https://go-review.googlesource.com/c/go/+/187217


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>