replace "Symfony2" with "Symfony"
minor #4205 replace "Symfony2" with "Symfony" (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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replace "Symfony2" with "Symfony"

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4203

Commits
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3ba3d27 replace "Symfony2" with "Symfony"