archive/tar: set Header.Mode in example

Creating a tar containing files with 0000 permission bits is
not going to be useful.

Change-Id: Ie489c2891c335d32270b18f37b0e32ecdca536a6
Reviewed-on: https://go-review.googlesource.com/3271Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/archive/tar/example_test.go

@@ -31,6 +31,7 @@ func Example() {
 	for _, file := range files {
 		hdr := &tar.Header{
 			Name: file.Name,
+			Mode: 0600,
 			Size: int64(len(file.Body)),
 		}
 		if err := tw.WriteHeader(hdr); err != nil {