fix fog types

fog takes either a number or a string: https://threejs.org/docs/index.html#api/en/scenes/Fog

https://github.com/react-spring/react-three-fiber/issues/503#issuecomment-643441952

src/scenes/Fog.d.ts

@@ -12,7 +12,7 @@ export interface IFog {
  */
 export class Fog implements IFog {
 
-	constructor( hex: number, near?: number, far?: number );
+	constructor( color: number | string, near?: number, far?: number );
 
 	name: string;