Bug 25155: (follow-up) Increase headings levels by 1

Bug 25155: (follow-up) Increase headings levels by 1

In the previous patch we reduced the headings levels by 2, but this was
one level too many as we already have an H1 in the navbar and so all
other heading sets should start at level 2 to semantically follow the
title heading.

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>