Bug 28380: Elasticsearch - correct 024aa in mappings (MARC21)

Bug 28380: Elasticsearch - correct 024aa in mappings (MARC21)

This patch corrects a typo in the mappings.yaml file

To test:
1) Apply patch
2) Reindex using rebuild_elasticsearch.pl -r

If you don't have access to a terminal (in a sandbox for example)
2a) Go to Administration > Search engine configuration, click "Reset mappings"
and confirm
2b) Then reindex

3) Search for a standard number found in 024$a using the Standard number index
in the advanced search

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>