Fix highlights in world-app.md

[TanX-009]

Fix the highlighting in the code block after 'Module Scoped Variables' in world-app. before fix,

...
class World {
  constructor(container) {        <--Highlighted Line
    camera = createCamera();      <--Highlighted Line
    scene = createScene();        <--Highlighted Line
    renderer = createRenderer();
  }
...

after fix,

...
class World {
  constructor(container) {
    camera = createCamera();      <--Highlighted Line
    scene = createScene();        <--Highlighted Line
    renderer = createRenderer();  <--Highlighted Line
  }
...

[looeee]

Thank you! :)