unix: remove redundant word in comment

Change-Id: I8b05b5cea86d41423dac9fd96a5000a6daa1e8c3
Reviewed-on: https://go-review.googlesource.com/c/sys/+/690816
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Mark Freeman <mark@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>