io/ioutil: fix typo in comment

R=r, r
CC=golang-dev
https://golang.org/cl/4592052

src/pkg/io/ioutil/ioutil.go

@@ -63,7 +63,7 @@ func WriteFile(filename string, data []byte, perm uint32) os.Error {
 	return err
 }
 
-// A dirList implements sort.Interface.
+// A fileInfoList implements sort.Interface.
 type fileInfoList []*os.FileInfo
 
 func (f fileInfoList) Len() int           { return len(f) }