Bug 30784: Rephrasing OPACMandatoryHoldDates pref slightly

Bug 30784: Rephrasing OPACMandatoryHoldDates pref slightly

TEST PLAN:

1. On the Admin page, navigate to Koha Administration > Global System
Preferences

2. Search for OPACMandatoryHoldDates

3. In the Value field, check that it now says On the "Placing a hold"
form, instead of opac-reserve form.

Sponsored by Catalyst IT

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>