Fix typo and clean up Cesium3DTileset specs
Fix typo and clean up Cesium3DTileset specs

likangning93/cesium,AnalyticalGraphicsInc/cesium,likangning93/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,CesiumGS/cesium,CesiumGS/cesium,likangning93/cesium,CesiumGS/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium