cmd/upspin: update doc.go, which was very old
cmd/upspin: update doc.go, which was very old

Just ran mkdoc.sh. Mechanical change.

Change-Id: Ic9440bad7d1e72a4711cb6592dec9244001c240e
Reviewed-on: https://upspin-review.googlesource.com/8106
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

upspin/upspin,upspin/upspin