Fixed FTP adapter arguments

As you can see from [this commit](https://github.com/KnpLabs/Gaufrette/commit/902de4417d02a61490ca0948db042e5ef91ebdec#src/Gaufrette/Adapter/Ftp.php) the arguments for the FTP adapter were changed to accept the directory, host and an array of options. But the "sonata.media.adapter.filesystem.ftp" service was being passed 7 arguments. This resulted in the `$options` array being set as the string for the directory, which was breaking the connection.