exp/types: fix package comment

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5695046

src/pkg/exp/types/types.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// PACKAGE UNDER CONSTRUCTION. ANY AND ALL PARTS MAY CHANGE.
-// Package types declares the types used to represent Go types.
+// Package types declares the types used to represent Go types
+// (UNDER CONSTRUCTION). ANY AND ALL PARTS MAY CHANGE.
 //
 package types