Add missing \n for "invalid optionusage" output

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: DHE <git@dehacked.net>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Jack Draak <jackdraak@gmail.com>
Signed-off-by: Kash Pande <kash@tripleback.net>
Closes #6203