[HttpFoundation] Fix return phpdoc

| Q             | A
| ------------- | ---
| Bug fix?      | tiny (for IDE autocompletion)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

What about using `self` or `static` keyword for this?