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/19551


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>