testing/iotest: adjust adverbial imprecision in package comment
testing/iotest: adjust adverbial imprecision in package comment

Fixes issue 2959.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5651053

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang