cmd/upbox: fix vet error
cmd/upbox: fix vet error

Change-Id: I38a3a03a7ac6eeb0d4d37c9125e54fb365c3322e
Reviewed-on: https://upspin-review.googlesource.com/5243
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin