Bug 31238: Fix typo in marc_subfields_structure.pl

Bug 31238: Fix typo in marc_subfields_structure.pl

After bug 29001 one is unable to save authorised value to frameworks
subfields. This is caused by minor typo in marc_subfields_structure.pl.

To test:
1. Edit e.g. default frameworks subfield 952$8
2. Add or change fields authorised value
3. Save your changes
=> note that on subfield listing there is no mention that 952$8 is linked to any authorised value
=> if you edit 952$8 authorised value input is empty
4. Apply this patch.
5. Repeat steps 1.-3.
=> authorised value should now be saved

Sponsored-by: Koha-Suomi Oy
Signed-off-by: Emmanuel Bétemps <e.betemps@gmail.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>