Fix mailgum header format for Reply-To,

[loduis]

In this moment is sent in lower case so: h:reply-to
You can see the article is h:Reply-To
https://help.mailgun.com/hc/en-us/articles/4401814149147-Adding-A-Reply-To-Address

[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.