Fix broken backward compatibility / snippet preview

With the refactor/extraction to it's own class of the snippet preview, the `wpseo_snippet` filter became broken for all implementations using the `description` as the expected array key was renamed from `desc` to `description` resulting in error notices like:
```
Notice: Undefined index: desc in /path/to/wpseo-video/class-wpseo-video-metabox.php on line 241
```

For our 'own' plugins, we can update the implementations of course, however, other plugins/themes might hook into the filter as well.