Update a-video.md (#3714)
Update a-video.md (#3714)

Making current and accurate the caveats at the bottom. Specifically, the video can work in the browser when not on the home screen (line removed); and since iOS 11,  user interaction required to play video.
aframevr/aframe,RSpace/aframe,aframevr/aframe,dmarcos/aframe,ngokevin/aframe,dmarcos/aframe,ngokevin/aframe,chenzlabs/aframe,RSpace/aframe,chenzlabs/aframe