cmd/upspin/signup.go: clarify NArg error
cmd/upspin/signup.go: clarify NArg error

On Windows PowerShell, apparently flags need to be quoted, which
can lead to mysterious Usage calls.  Add a clarification.

This was prompted by issue #368.

Change-Id: I751c8f946e3a25555d74fd758b68ec3a8300cf3e
Reviewed-on: https://upspin-review.googlesource.com/9220
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin