config system blueprint assets.collections type change to multilevel

Since the upgrade to Grav 1.3.3 the `assets.collections` type changed from `array` to `multilevel`.
I don't know if this was supposed, but it looks really odd and it leads to an error if new elements are not added correctly.


I changed the type back to `array` in `/system/blueprints/config/system.yaml` but if not corrected, the next update will overwrite this.

Open /system/blueprints/config/system.yaml and inspect the assets.collections definition, starting with the reported type change from array to multilevel. Restore the intended type and verify that the configuration no longer produces the reported error for newly added elements and will not be overwritten on the next update.

Written by the indexing model from the issue text.