title: Symfony4 "object not found by the @ParamConverter annotation" 404 error
body: <p>I discover Symfony4 with similar blog sample like describe in <a href="https://symfony.com/doc/current/routing.html" rel="noreferrer">https://symfony.com/doc/current/routing.html</a>
  Then I added a new route to add /blog/about page. So a part of code in my <em>src/Controller/BlogController.php</em> is:</p>
  
  <pre><code>/**
   * @Route("/blog/{id}", name="blog_show")
   */
  public function show(Description $article) {
      return $this-&gt;render('blog/show.html.twig', [
          'article' =&gt; $article,
      ]);
  }
  
  /**
   * @Route("blog/about", name="about")
   */
  public function about() {
      return $this-&gt;render('blog/about.html.twig', [
          'copyright' =&gt; "GLPI 3",
      ]);
  }
  </code></pre>
  
  <p>and when I run locahost:8000/blog/about, it returns me a 404 error :<br>
  <em>App\Entity\Description <strong>object not found by the @ParamConverter annotation</em></strong></p>