cmd/toolstash: fix format string arguments

Found by vet.

Follow-up to comments on CL 32682.

Change-Id: I76391e02b9991bc480404f634ea809e75645e9dc
Reviewed-on: https://go-review.googlesource.com/32918
Reviewed-by: Alan Donovan <adonovan@google.com>
1 file changed