Bug 33069: Fix error in MARC download for OPAC lists

Bug 33069: Fix error in MARC download for OPAC lists

There was a code typo causing an error when you tried
to download a list in MARC format:

Not a CODE reference at /kohadevbox/koha/opac/opac-downloadshelf.pl line 93

To test:
* In the OPAC: create a new list, public or private, with some records
* Open the list
* Try to download the list as MARC
* Verify you get an error
* Apply patch
* Verify you now get the expected download file
* Sign off :)

Co-authored-by: Kévin AYRAULT <kevin.ayrault87@gmail.com>
Signed-off-by: Magnus Enger <magnus@libriotech.no>
Nasty error before the patch; nice, tasty MARC download after.
Signed-off-by: Hinemoea Viault <hinemoea.viault@inlibro.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>