Bug 25239: Fix semantic heading for opac-reserve.tt

Bug 25239: Fix semantic heading for opac-reserve.tt

Prior to this patch, the opac-reserve contained semantically incorrect headings.

Test plan
1/ Perform a search which yields some results in the OPAC
2/ Attempt to place a hold on an item.
3/ On the resulting screen, note that the headings are:
   H1 (Koha page title) > H3 (Confirm holds for) > H5 (Hold levels)
4/ Apply this patch and reload the page
5/ Re-inspect the elements in question note the heading semantics are now:
   H1 (Title of page) > H2 (Confirm holds for) > H3 (Hold levels)
6/ Note the appearance of the box is still reasonable
7/ Signoff

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>