Fix ordering of arguments to Matrix3.getElementIndex in documentation.
Fix ordering of arguments to Matrix3.getElementIndex in documentation.

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