[docs] fix asset management docs for image load event (fixes #2057)
[docs] fix asset management docs for image load event (fixes #2057)

ngokevin/aframe,RSpace/aframe,wizgrav/aframe,weddingdj/aframe,darkwing/aframe,jaxmolina/ar,dmarcos/aframe,darkwing/aframe,fernandojsg/aframe,aframevr/aframe,mkungla/aframe,weddingdj/aframe,RSpace/aframe,chenzlabs/aframe,ngokevin/aframe,wizgrav/aframe,fernandojsg/aframe,dmarcos/aframe,chenzlabs/aframe,googlecreativelab/aframe,jaxmolina/ar,aframevr/aframe,mkungla/aframe,googlecreativelab/aframe