Promos - Edit content regarding not using a heading within a promo

After recent feedback and resulting usability testing with screen reader users - it is now recommended to wrap the promo link inside a `h3` when in the context of an index page such as BBC News.

The content below requires some updating to ensure this research insight is accounted for in the tech docs.

https://bbc.github.io/gel/components/promos/#aside-info-headings-in-headlines