[book] [routing] fixed a note that wasn't properly updated

Previously, the code used the generate() method of the service and
the note explained the generateUrl() method of the base controller.
Then, the example was updated to use the generateUrl(), but the
note kept explaining the generateUrl(). This change is just about
explaining in the note the alterantive generate() method of the service.

book/routing.rst

@@ -1168,12 +1168,15 @@ route. With this information, any URL can easily be generated::
 
 .. note::
 
-    In controllers that extend Symfony's base
+    In controllers that don't extend Symfony's base
     :class:`Symfony\\Bundle\\FrameworkBundle\\Controller\\Controller`,
-    you can use the
-    :method:`Symfony\\Bundle\\FrameworkBundle\\Controller\\Controller::generateUrl`
-    method, which calls the router service's
-    :method:`Symfony\\Component\\Routing\\Router::generate` method.
+    you can use the ``router`` service's
+    :method:`Symfony\\Component\\Routing\\Router::generate` method::
+
+        $url = $this->get('router')->generate(
+            'blog_show',
+            array('slug' => 'my-blog-post')
+        );
 
 In an upcoming section, you'll learn how to generate URLs from inside templates.