Grammar fix

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none

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BTW, I was not sure if I need to follow [this](http://symfony.com/doc/current/contributing/documentation/overview.html#why-should-i-use-the-oldest-maintained-branch-instead-of-the-master-branch) rule while creating a PR. Most of open/closed PRs I've checked are master-master.