Minor tweaks in the testing article

Author: javiereguiluz

testing.rst

@@ -268,7 +268,7 @@ the container is stored in ``static::getContainer()``::
             $newsletterGenerator = $container->get(NewsletterGenerator::class);
             $newsletter = $newsletterGenerator->generateMonthlyNews(...);
 
-            $this->assertEquals(..., $newsletter->getContent());
+            $this->assertEquals('...', $newsletter->getContent());
         }
     }