[README] Remove the link to the article in the "tutorial" section because it uses deprecated code

[matthias-prevost]

The article and the video which are linked in the "tutorial" section uses deprecated code and could be removed to avoid problems when installing the package (if we don't see the instructions lower in the README like me 😅).
The ressources I am mentioning are the following :

• article
• video

Or maybe you could just add a warning to introduce the tutorial section that says the code is not up to date, because these ressources still contain interesting thing (like the creation of a dart package in the video for instance).