Fix use mistakes.
Fixing a mistake
bug #4120 Fix use mistakes (mbutkereit)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4120).

Discussion
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Fix use mistakes

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

On http://symfony.com/doc/current/components/config/resources.html at "new YamlUserLoader", we have also a missing "use" statement. But i'm not sure if we need there one.

Commits
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7016c82 Fixing a mistake
4353d1f Fix use mistakes.