Bug 14636: Follow-up - fix typo in system preference name

Bug 14636: Follow-up - fix typo in system preference name

The preference is named "marcflavour" not "marcflavor".

Without this change, search will always use "copyrightyear"
and break search by publication date for UNIMARC.

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 99a1d7f18a43a089d49d36b167cc1537fa058f79)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>