As the documentation states, you need to edit the file qa-config.php (copied from qa-config-example.php), not the file index.php shown in your screenshot.
In qa-config.php you should find the following lines
define('QA_MYSQL_HOSTNAME', '127.0.0.1');
define('QA_MYSQL_USERNAME', 'your-mysql-username');
define('QA_MYSQL_PASSWORD', 'your-mysql-password');
define('QA_MYSQL_DATABASE', 'your-mysql-db-name');
and fill in your database credentials.