Bug 16717: Improve variable naming

Bug 16717: Improve variable naming

$category was a categorycode, to help the readability it's better to
name it $categorycode or assign a Category to $category

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit f8b73deaec5c7f55d66d52c2ca05d419f4a7c178)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
(cherry picked from commit 8ebe3a0a42b8b63a3ef8b1486a30cd3b335d12b9)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>