Fix coding standards

I noticed an error was produced when checking for coding standard violations:

phpcs --standard=DrupalPractice .

FILE: /var/www/html/drupal2/modules/social_auth_uber/src/UberAuthManager.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 33 | ERROR | The $_GET super global must not be accessed directly; inject the request.stack
    |       | service and use $stack->getCurrentRequest()->query->get('code') instead
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Time: 82ms; Memory: 6MB