improve GroundPolylinePrimitive doc example
improve GroundPolylinePrimitive doc example

likangning93/cesium,CesiumGS/cesium,likangning93/cesium,CesiumGS/cesium,likangning93/cesium,YonatanKra/cesium,CesiumGS/cesium,CesiumGS/cesium,progsung/cesium,CesiumGS/cesium,YonatanKra/cesium,AnalyticalGraphicsInc/cesium,progsung/cesium,YonatanKra/cesium,likangning93/cesium,YonatanKra/cesium,AnalyticalGraphicsInc/cesium,likangning93/cesium