Fix an issue in the create_empty_index rake task

ee/lib/tasks/gitlab/elastic.rake

@@ -68,7 +68,7 @@ namespace :gitlab do
       helper = Gitlab::Elastic::Helper.new(target_name: args[:target_name])
       index_name = helper.create_empty_index(with_alias: with_alias, options: options)
 
-      helper.create_migrations_index unless helper.index_exists?(index_name: migrations_index_name)
+      helper.create_migrations_index unless helper.index_exists?(index_name: helper.migrations_index_name)
       ::Elastic::DataMigrationService.mark_all_as_completed!
 
       puts "Index '#{index_name}' has been created.".color(:green)