Update groundMaterial shading
Update groundMaterial shading

Suppresses the following warning: `THREE.MeshLambertMaterial: .shading has been removed. Use the boolean .flatShading instead.`

`.shading` has been removed and is now `.flatShading`, see: https://threejs.org/docs/index.html#api/materials/Material
feiss/aframe-environment-component,feiss/aframe-environment-component