Bug 31396: Fix incorrect attribute access in opac-detail.pl

Bug 31396: Fix incorrect attribute access in opac-detail.pl

To test:
1. Have an item with a callnumber defined
2. Have OPACShelfBrowser enabled
3. Open the record detailed view in the OPAC
4. Click on 'Browse shelf'
=> FAIL: It explodes
5. Apply this patch
6. Repeat 4
=> SUCCESS: It works!
7. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>