pmcstat: fix the usage message for -t

A slight mix-up of the flags means this case isn't triggered when it
should be. Now, pmcstat -s event -t 1234 will print the correct
error message.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit c4473add1d3d588d8747cab86796d2f2d4c8847c)