archive/tar: fix inconsistent namespace usage in example

This fixes some example code in the tar package documentation, which
first refers to tar.NewWriter and then to Header, which is inconsistent
because NewWriter and Header are both in the tar namespace.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6595050

src/pkg/archive/tar/writer.go

@@ -27,7 +27,7 @@ var (
 //
 // Example:
 //	tw := tar.NewWriter(w)
-//	hdr := new(Header)
+//	hdr := new(tar.Header)
 //	hdr.Size = length of data in bytes
 //	// populate other hdr fields as desired
 //	if err := tw.WriteHeader(hdr); err != nil {