Update building-an-advanced-scene.md
Update building-an-advanced-scene.md

Superfluous variable, changed in document, needs to be changed in the codepen but I imagine you'd want to do that through your account.
weddingdj/aframe,aframevr/aframe,fernandojsg/aframe,dmarcos/aframe,jaxmolina/ar,jzitelli/aframe,msimpson/aframe,wizgrav/aframe,chenzlabs/aframe,chenzlabs/aframe,jaxmolina/ar,googlecreativelab/aframe,jzitelli/aframe,ngokevin/aframe,RSpace/aframe,MatthewShotton/aframe,darkwing/aframe,MatthewShotton/aframe,aframevr/aframe,dmarcos/aframe,msimpson/aframe,fernandojsg/aframe,darkwing/aframe,mkungla/aframe,ngokevin/aframe,googlecreativelab/aframe,RSpace/aframe,mkungla/aframe,wizgrav/aframe,weddingdj/aframe