minor symfony#17442 Fix markdown typo (welcoMattic)

This PR was merged into the 2.7 branch.

Discussion
----------

Fix markdown typo

There was too much backticks before Config paragraph

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

Commits
-------

bd70f24 Fix markdown typo

Author: Tobion

UPGRADE-3.0.md

@@ -19,7 +19,7 @@ UPGRADE FROM 2.x to 3.0
  * The `DebugUniversalClassLoader` class has been removed in favor of
    `DebugClassLoader`. The difference is that the constructor now takes a
    loader to wrap.
-   ```
+
 
 ### Config