[Filesystem] Fixed fabbot report about 'unix' comment

This is a minor coding standards PR. (fabbot warning)

--> http://hook.fabbot.io/report/symfony/symfony/9867/d136555fc79010850bce7c84f5d1b5240053fe42

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [no]
| Fixed tickets | [Coding standards]
| License       | MIT
| Doc PR        | [NA]