Fix structure template id and structure spawn height

[Silverminer007]

This PR fixes two minor issues concerning structure spawning.

• The Template (nbt file) isn't loaded -> fix wrong resource location
• The Structure spawns floating in the air, even tho it should be an underground structure -> fix height calculation

New height calculation: ((WORLD_SURFACE - WORLD_MIN_Y) / 2) + WORLD_MIN_Y

The structure spawns always in the middle between the surface and the min y level in the overworld