Clarify a comment

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The previous comment was lying since [`collectViewVariables()`](https://github.com/symfony/symfony/blob/cffb64fd931d167c905f9bbecdb400e71cb3c850/src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php#L165) doesn't really call the [`buildPreliminaryFormTree()`](https://github.com/symfony/symfony/blob/cffb64fd931d167c905f9bbecdb400e71cb3c850/src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php#L186) nor the [`buildFinalFormTree()`](https://github.com/symfony/symfony/blob/cffb64fd931d167c905f9bbecdb400e71cb3c850/src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php#L196).