cmd/bent: fix cpuprofile command

Change-Id: Ia20ad3e7daa4741a9a3f8675c782f647595f2c5e
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/614775
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>