Bug 19313: Fix typo in in unimarc_field_130.pl value builder

Bug 19313: Fix typo in in unimarc_field_130.pl value builder

This patch fixes a simple typo

Test plan:
1) Go to Administration -> MARC bibliographic framework
2) Select a framework and add plugin unimarc_field_130.pl to 130 field
3) Open cataloguing screen
4) Go to 130 field and check if for subfield a shows value for
   'unknown' select it and close the window, check on value on 130$a at
   position 8 to have the value 'd'.
5) Repeat step 4 and select the value 'mixed emulsion' close the plugin
   window and check at position 8 to have the value 'u'
6) Apply the patch
7) Go to field 130, open the plugin window and select value 'unknown'
   close the window and check at position 8 to have the value 'u'
8) Go to field 130, open the plugin window and select value 'mixed
   emulsion', close the window and check at position 8 to have the
   value 'v'

https://www.ifla.org/files/assets/uca/unimarc_updates/BIBLIOGRAPHIC/b_130_update2017.pdf

Mentored-by: Andreas Roussos
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>