minor symfony#25870 Formatting fix in upgrade 3.0 document (Piotr Błasiak)

This PR was merged into the 2.8 branch.

Discussion
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Formatting fix in upgrade 3.0 document

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

Fixes formatting in upgrade file.
This fix was already done in symfony#22436 but applied to 3.2.
Formatting error still exists in 2.8, 3.0, 3.1. I don't know if this was intentional.

Commits
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dc6d5e1 Formatting fix in upgrade 3.0 document

Author: fabpot

UPGRADE-3.0.md

@@ -1098,7 +1098,7 @@ 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)`