cmd/asm: fix format string so vet doesn't complain

cmd/asm: fix format string so vet doesn't complain

Fixes #70309

Change-Id: I4a3e27e89bdfda66d64f2efbb4c08a5ddde34a52
Reviewed-on: https://go-review.googlesource.com/c/go/+/626040
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>