archive/tar: add Copyright notice to example_test.go

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/7278050

src/pkg/archive/tar/example_test.go

+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package tar_test
 
 import (