Bug 25236: Fix semantic heading for opac-facets.inc

Bug 25236: Fix semantic heading for opac-facets.inc

Prior to this patch, the opac-facets side navigation box contained
semantically incorrect headings.

Test plan
1/ Perform a search which yields some results in the OPAC
2/ Inspect the 'Refine your search' elements on the left side of the
screen. Note that the headings are H1 (Koha page title) > H4 (Refine
your search) > H5 (Headings within 'Refine your search' box)
3/ Apply this patch and follow
https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client
4/ Reload the page (Flushing the cache to ensure the CSS change load)
5/ Re-inspect the 'Refine your search' elements and note the heading
semantics are now H1 (Title of page) > H2 (Refine your search) > H3
(Headings within 'Refine your search')
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>