Add a missing possible returned type in comment

[Thaldos]

Hello,

The method http-foundation\InputBag.php > get(string $key, $default = null) can return an array, but our IDE notice that it is not possible because the array type is missing in the comment of this function.

This PR is to add the missing array type in comment, to let our IDE know that get can return an array.

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony repository. It is used by Composer to allow developers to depend on specific Symfony components.

If you want to contribute, you should instead open a pull request on the main repository:

https://github.com/symfony/symfony

Thank you for your contribution!

PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply.