Fix code typo in SceneGraph manual page

[MeirArani]

Description

The sample code in the manual for the Scene Graph fundamentals has a code typo that was causing me some grief. Particularly this chunk of code:

{
  const color = 0xFFFFFF;
  const intensity = 3;
  const light = new THREE.PointLight(color, intensity);
  scene.add(light);
}

had the wrong intensity value (should be 500, not 3). It took me some time to realize why I couldn't get my example matching up with the docs.

I updated the docs to match the correct value in all the areas I could spot the issue.