Bug 9118: Do not show option to create exception on range for unique holiday

Bug 9118: Do not show option to create exception on range for unique holiday

When an unique holiday is edited one can choose
option 'Generate exceptions on a range of dates.'
Exceptions are meant to be used with repeating
holidays and it makes no sense to have option to
be able to create one for an unique holiday.

This patch hides exception on range option when
editing an unique holiday. Also fixes a minor
spelling error.

To test:
1. Create unique holiday and edit it.
2. Note that option to create exception on range is shown.
3. Apply patch.
=> Option is now hidden.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>