Change 'Nonprinting Characters' to 'Non-printing Characters'

Change-Id: I599b2bc3fe80805b87cbb83c1dc74eba85799105
Reviewed-on: https://gerrit.libreoffice.org/9884Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>