bug #4153 [Reference] fix namespace in Expression constraint (xabbuh)

This PR was merged into the 2.4 branch.

Discussion
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[Reference] fix namespace in Expression constraint

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4+
| Fixed tickets |

Commits
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1deabde [Reference] fix namespace in Expression constraint

reference/constraints/Expression.rst

@@ -73,7 +73,7 @@ One way to accomplish this is with the Expression constraint:
     .. code-block:: php-annotations
 
         // src/Acme/DemoBundle/Model/BlogPost.php
-        namespace Acme\DemoBundle\Model\BlogPost;
+        namespace Acme\DemoBundle\Model;
 
         use Symfony\Component\Validator\Constraints as Assert;