replace Symfony2 with Symfony
minor #4652 replace Symfony2 with Symfony (xabbuh)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4652).

Discussion
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replace Symfony2 with Symfony

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

Commits
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224c283 replace Symfony2 with Symfony