Ground color setter missed underscore

proceduralTexturesLibrary/src/grass/babylon.grassProceduralTexture.ts

@@ -38,8 +38,8 @@ module BABYLON {
         }
 
         public set groundColor(value: Color3) {
-            this.groundColor = value;
+            this._groundColor = value;
             this.updateShaderUniforms();
         }
     }
-}
+}