Fix link to fortrabbit article

[sebpacz]

The old link https://help.fortrabbit.com/install-symfony-4-uni doesn't work (it shows 404 page). On the new page https://help.fortrabbit.com/install-symfony-5 you can find a guide for Symfony 5, but it's also compatible with Symfony 4:

If you are using Symfony 4, you can safely follow this guide, given Symfony 5 did not break compatibility changes.

[javiereguiluz]

Thanks Sebastian.