testing/iotest: adjust adverbial imprecision in package comment

Fixes #2959.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5651053

src/pkg/testing/iotest/reader.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package iotest implements Readers and Writers useful only for testing.
+// Package iotest implements Readers and Writers useful mainly for testing.
 package iotest
 
 import (