fix: Stronghold EntryDoor GRATES

[xiej2520]

nit, the enum name matters for StrongholdPiece serialization:

	abstract static class StrongholdPiece extends StructurePiece {
		// ...
		@Override
		protected void writeNbt(NbtCompound nbt) {
			nbt.putString("EntryDoor", this.entranceType.name());
		}
		// ...
		public static enum EntranceType {
			OPENING,
			WOOD_DOOR,
			GATES,
			IRON_DOOR;
		}

Stronghold.dat:

{
  data: {
    Features: {
      "[289,39]": {
        BB: [I;4529,8,574,4696,38,707],
        ChunkZ: 39,
        id: "Stronghold",
        Children: [
          {
            BB: [I;4626,28,626,4630,38,630],
            EntryDoor: "OPENING",
            id: "SHStart",
            GD: 0,
            Source: 1b,
            O: 3
          },
          {
            BB: [I;4631,26,623,4641,34,632],
            leftLow: 1b,
            EntryDoor: "IRON_DOOR",
            id: "SH5C",
            GD: 1,
            rightHigh: 0b,
            leftHigh: 1b,
            rightLow: 0b,
            O: 3
          },
          {
            BB: [I;4642,26,627,4648,30,631],
            Left: 0b,
            EntryDoor: "GRATES",
            Right: 0b,
            id: "SHS",
            GD: 2,
            O: 3
          },
...