net/http: update bundle command

This is the bundle command's new usage and new output header,
after CL 19428.

Actually running this command would work but would bring in
a newer x/net/http2 that we don't want yet.

Change-Id: Ic6082ca00102a2df1f7632eebf9aca41fdcdb444
Reviewed-on: https://go-review.googlesource.com/19551Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>

src/net/http/h2_bundle.go

-// Code generated by golang.org/x/tools/cmd/bundle command:
-//   $ bundle golang.org/x/net/http2 net/http http2
+// Code generated by golang.org/x/tools/cmd/bundle.
+//go:generate bundle -o h2_bundle.go -prefix http2 -import golang.org/x/net/http2/hpack=internal/golang.org/x/net/http2/hpack golang.org/x/net/http2
 
 // Package http2 implements the HTTP/2 protocol.
 //