Fix display name (and slug?) of Aerobellum

[zzufx]

This edit also changes the slug. I'm guessing https://github.com/MCResourcePile/overcast-maps/tree/master/maps/arobellum 's folder name also needs to be changed, but I lack the knowledge in how to do so.

[CrazyisCreeps]

Hey, I totally missed this! Thanks for the pull request, I'll go along and update the map folder's name alongside with your changes