minor #12458 Update UPGRADE-2.6.md (perajovic)

This PR was submitted for the master branch but it was merged into the 2.6 branch instead (closes #12458).

Discussion
----------

Update UPGRADE-2.6.md

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | -
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
-------

450e341 Update UPGRADE-2.6.md

UPGRADE-2.6.md

@@ -357,9 +357,9 @@ The component and the bundle are new to Symfony 2.6. We encourage you
 to enable the bundle in your `app/AppKernel.php` for the *dev* or *test*
 environments. Just add this line before loading the `WebProfilerBundle`:
 
-    ```php
-    $bundles[] = new Symfony\Bundle\DebugBundle\DebugBundle();
-    ```
+```php
+$bundles[] = new Symfony\Bundle\DebugBundle\DebugBundle();
+```
 
 Then enjoy dumping variables by calling `dump($var)` anywhere in your PHP
 and `{% dump var %}` or `{{ dump(var) }}` in Twig. Dumps are displayed