Update README.md

change "var" to "let"

README.md

@@ -27,8 +27,8 @@ This code creates a scene, a camera, and a geometric cube, and it adds the cube
 ```javascript
 import * as THREE from './js/three.module.js';
 
-var camera, scene, renderer;
-var geometry, material, mesh;
+let camera, scene, renderer;
+let geometry, material, mesh;
 
 init();
 animate();