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/2139Reviewed-by: Alan Donovan <adonovan@google.com>

src/testing/iotest/logger.go

@@ -48,7 +48,7 @@ func (l *readLogger) Read(p []byte) (n int, err error) {
 
 // NewReadLogger returns a reader that behaves like r except
 // that it logs (using log.Print) each read to standard error,
-// printing the prefix and the hexadecimal data written.
+// printing the prefix and the hexadecimal data read.
 func NewReadLogger(prefix string, r io.Reader) io.Reader {
 	return &readLogger{prefix, r}
 }