[Doc] Update the syntax of doc images

[javiereguiluz]

This will probably fix the following error reported while building docs:

Errors for ux-twig-component / 2.x

Build errors from "2022-05-02 01:21:40"
Missing image file "https://github.com/symfony/ux-twig-component/blob/2.x/alert-example.png?raw=true" in "index"

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony UX repository. It is used by Composer to allow developers to depend on specific Symfony UX packages.

If you want to contribute, you should instead open a pull request on the main repository:

https://github.com/symfony/ux

Thank you for your contribution!

PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply; only features should be submitted against the main branch.

[javiereguiluz]

Closing in favor of symfony/ux#318