added poster attribute to video snippet

The "thumb" parameter is not named appropriately. It does not set a thumbnail image but rather a fallback image for browsers that can not handle the HTML5 video element. As a result modern browsers will have no use for this parameter.

This commit adds the "thumb" URL to the "poster" attribute and thus displays a thumbnail before the video starts to play.

The solution is not perfect as "thumb" is still misleading. I think it would be best to introduce two new variables named "poster" and "fallback" that correspond to their actual function in the code. 

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/video