Fix the migration that adds the idx_container_scanning_findings index

changelogs/unreleased/patch-migration.yml

+---
+title: Fix rollback portion of migration that adds temporary index for container scanning findings
+merge_request: 44593
+author:
+type: fixed

db/migrate/20200820105408_add_index_to_container_scanning_findings.rb

@@ -15,6 +15,6 @@ class AddIndexToContainerScanningFindings < ActiveRecord::Migration[6.0]
   end
 
   def down
-    remove_concurrent_index_by_name(:project_registry, INDEX_NAME)
+    remove_concurrent_index_by_name(:vulnerability_occurrences, INDEX_NAME)
   end
 end