testing/iotest: fix copy/paste error in comment

Reported via unsupported GitHub pull request:
https://github.com/golang/go/pull/9436/

Change-Id: I12b00e8ccac700bb36b200196e2867fcc863fdf1
Reviewed-on: https://go-review.googlesource.com/2139
Reviewed-by: Alan Donovan <adonovan@google.com>