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

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

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

Test plan
0/ Enable 'OpacTopIssues'
1/ Navigate to the top issues page via the 'Most popular' link
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)
3/ Apply this patch
4/ Reload the page
5/ Re-inspect the 'Refine your search' elements and note the heading
   semantics are now H1 (Title of page) > H2 (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>