Bug 32829: Fix cataloguing/value_builder/unimarc_field_115b.pl

Bug 32829: Fix cataloguing/value_builder/unimarc_field_115b.pl

Use event parameter

This is part of the refactoring happening in bug 30975

Test plan:
1. Verify that the plugin continues to work as before

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit a2edbc0529cece0e0cca0f3f2540132d19a5ebb3)
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
(cherry picked from commit a538c7515c7b3bc69aca343831d1a3004df5d246)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>