minor symfony#20448 Added single quotes for upgrade guides. (Aliance)

This PR was merged into the 2.8 branch.

Discussion
----------

Added single quotes for upgrade guides.

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | –
| License       | MIT
| Doc PR        | –

Commits
-------

2016031 Added single quotes for upgrade guides.

Author: fabpot

UPGRADE-2.8.md

@@ -628,7 +628,7 @@ HttpFoundation
    After:
 
    ```php
-   $request->query->get('foo')[bar];
+   $request->query->get('foo')['bar'];
    ```
 
 Routing
@@ -657,4 +657,4 @@ Routing
 
    // url generated in @router service
    $router->generate('blog_show', array('slug' => 'my-blog-post'), UrlGeneratorInterface::ABSOLUTE_URL);
-   ```
+   ```

UPGRADE-3.0.md

@@ -1750,5 +1750,5 @@ UPGRADE FROM 2.x to 3.0
    After:
 
    ```php
-   $request->query->get('foo')[bar];
+   $request->query->get('foo')['bar'];
    ```