minor #318 [Doc] Update the syntax of doc images (javiereguiluz)

This PR was merged into the 2.x branch.

Discussion
----------

[Doc] Update the syntax of doc images

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| Tickets       | -
| License       | MIT

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"
```

Commits
-------

c7e3349 [Doc] Update the syntax of doc images

src/TwigComponent/src/Resources/doc/index.rst

@@ -39,7 +39,7 @@ Done! Now render it wherever you want:
 
 Enjoy your new component!
 
-.. figure:: https://github.com/symfony/ux-twig-component/blob/2.x/alert-example.png?raw=true
+.. image:: images/alert-example.png
    :alt: Example of the AlertComponent
 
    Example of the AlertComponent