LP#1233757 Correct Spelling Error in Checkout Override Exception

Correcting a few misspellings of "overridden" in the staff client
override exception.

Signed-off-by: Elliot Voris <evoris@slcconline.edu>
Signed-off-by: Kathy Lussier <klussier@masslnc.org>
Signed-off-by: Ben Shum <bshum@biblio.org>