minor symfony#22436 Fixed formatting in Security section (in UPGRADE-3.0.md) (zelazowy)

This PR was merged into the 3.3-dev branch.

Discussion
----------

Fixed formatting in Security section (in UPGRADE-3.0.md)

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

Added space (yeah...) to Security section fixing text and code format.

Commits
-------

3bde16c Fixed formatting in Security section

Author: fabpot

UPGRADE-3.0.md

@@ -1144,7 +1144,7 @@ UPGRADE FROM 2.x to 3.0
            'http_digest' => array('secret' => '%secret%'),
        ),
    ));
-  ```
+   ```
 
  * The `AbstractVoter` class was removed. Instead, extend the new `Voter` class,
    introduced in 2.8, and move your voting logic to the to the `supports($attribute, $subject)`