Bug 38173: Fix description of koha-dump --exclude-indexes

Bug 38173: Fix description of koha-dump --exclude-indexes

To test:
- Look at debian/scripts/koha-dump
- The description of --exclude-indexes reads:
  "Include Zebra indexes on the backup"
- Apply the patch
- Look at the file again. The description is now:
  "Exclude Zebra indexes from the backup"

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Phil Ringnalda <phil@chetcolibrary.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>