[DX] Added line feed to container alias description (text descriptor)

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | N/A

This PR just adds a line feed at the end of the alias service description delivered by the ContainerDebugCommand. Tests are passing because expected outputs are trimmed before comparison (which IMO is not a really good thing, but hey...)
There's only one use of ``PHP_EOL`` that's not testing related in the whole symfony/symfony repo. I don't remember why but I think that was a choice made by the the core at some point.