Fix typo (#162)

Co-authored-by: Torsten Knauf <[email protected]>

threejs/lessons/threejs-scenegraph.md

@@ -225,7 +225,7 @@ objects.push(earthMesh);
 ```
 
 Again we added more invisible scene graph nodes. The first, an `Object3D` called `earthOrbit`
-and added both the `earthMesh` and the `moonOrbit` to it, also new. We then added the `mooMesh`
+and added both the `earthMesh` and the `moonOrbit` to it, also new. We then added the `moonMesh`
 to the `moonOrbit`. The new scene graph looks like this.
 
 <img src="resources/images/scenegraph-sun-earth-moon.svg" align="center">