Wrong #format key for export_icon theme cause errors

This change is correct. It should be #format.

views_data_export/src/Plugin/views/style/DataExport::attachTo() has this code:

    // Add the icon to the view.
    $format = $this->displayHandler->getContentType();
    $this->view->feedIcons[] = [
      '#theme' => 'export_icon',
      '#url' => $url,
      '#format' => mb_strtoupper($format),
      '#theme_wrappers' => [
        'container' => [
          '#attributes' => [
            'class' => [
              Html::cleanCssIdentifier($format) . '-feed',
              'views-data-export-feed',
            ],
          ],
        ],
      ],
    ];

Patch is working. Update title. Still RTBC !