Bug 36615: Terminology: use 'on hold' instead of 'reserved' in OPAC self checkout

Bug 36615: Terminology: use 'on hold' instead of 'reserved' in OPAC self checkout

This patch changes the term 'reserved' to 'on hold' in the OPAC self
checkout feature.

To test:
1. Apply patch
2. Place a hold for a patron
3. Enable OpacTrustedCheckout
4. Log into the OPAC as another patron
5. Click Self checkout at the top of the page
6. Paste the item's barcode in the text input
7. Click 'Submit'
8. Make sure the warning sentence is correct and there are no spelling
   errors

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>