Fix typo

threejs/lessons/threejs-scenegraph.md

@@ -113,7 +113,7 @@ Since we added the `sunMesh` to the `objects` array it will rotate.
 
 {{{example url="../threejs-scenegraph-sun.html" }}}
 
-Now let's add an the earth.
+Now let's add in the earth.
 
 ```js
 const earthMaterial = new THREE.MeshPhongMaterial({color: 0x2233FF, emissive: 0x112244});
@@ -387,4 +387,4 @@ to make something move and rotate the way you want. For example without a scene
 computing the motion of the moon or where to put the wheels of the car relative to its
 body would be very complicated but using a scene graph it becomes much easier.
 
-[Next up we'll go over materials](threejs-materials.html).
+[Next up we'll go over materials](threejs-materials.html).