Есть 2 варианта:
Первый через MapEntity:
#[Route('/article/{slug}', name: 'blog_article', methods: ['GET'])]
public function article(
    #[MapEntity(mapping: ['slug' => 'slug'])] Article $article
): Response {
    return $this->render('blog/article/show.html.twig', [
        'article' => $article,
    ]);
}

Второй вариант через doctrine.yaml:
doctrine:
    orm:
        controller_resolver:
            auto_mapping: true
тогда:
#[Route('/article/{slug}', name: 'blog_article_show', methods: ['GET'])]
public function article(
    Article $article
): Response {
    return $this->render('blog/article/show.html.twig', [
        'article' => $article,
    ]);
}