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/+/187217Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/help/help.go

@@ -63,7 +63,7 @@ Args:
 		// helpSuccess is the help command using as many args as possible that would succeed.
 		helpSuccess := "go help"
 		if i > 0 {
-			helpSuccess = " " + strings.Join(args[:i], " ")
+			helpSuccess += " " + strings.Join(args[:i], " ")
 		}
 		fmt.Fprintf(os.Stderr, "go help %s: unknown help topic. Run '%s'.\n", strings.Join(args, " "), helpSuccess)
 		base.SetExitStatus(2) // failed at 'go help cmd'