minor #25554 [4.0][Console] Fix upgrade guide example (romainneutron)

This PR was submitted for the 4.0 branch but it was merged into the 3.4 branch instead (closes #25554).

Discussion
----------

[4.0][Console] Fix upgrade guide example

| Q             | A
| ------------- | ---
| Branch?       | 4.0
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Otherwise
```
Symfony\Component\DependencyInjection\Exception\InvalidArgumentException: Namespace prefix must end with a "\": AppBundle\Command.
```

Commits
-------

d6fea56 [Command] Fix upgrade guide example

UPGRADE-4.0.md

@@ -627,7 +627,7 @@ HttpKernel
        # ...
 
        # explicit commands registration
-       AppBundle\Command:
+       AppBundle\Command\:
            resource: '../../src/AppBundle/Command/*'
            tags: ['console.command']
    ```