MediaBundle Elfinder config update.

Update's the config description to work with the new Elfinder config (as changed in https://github.com/helios-ag/FMElfinderBundle/commit/97c884a4fd0ab16066be041f65b2e38f69af6b89#diff-04c6e90faac2675aa89e2176d2eec7d8R141 )