shorter amazon S3 url

[bookstack] / config / setting-defaults.php

<?php

/**
 * The defaults for the system settings that are saved in the database.
 */
return [

    'app-editor' => 'wysiwyg',
    'app-color'  => '#0288D1',
    'app-color-light' => 'rgba(21, 101, 192, 0.15)'

];