cmd/upspin: clarify wording on rotating keys error message
cmd/upspin: clarify wording on rotating keys error message

Change-Id: Id3db73fd6f272eb4aaa25409933b6789dc9ab067
Reviewed-on: https://upspin-review.googlesource.com/8762
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin