[Doc] Use Markdown syntax highlighting

A clean version of the https://github.com/symfony/symfony/pull/11999

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 
Ok ! I'll push an update soon.

Le 29 sept. 2014 à 18:58, Christophe Coevoet <notifications@github.com> a écrit :

> In UPGRADE-2.1.md:
> 
> >      test.
> >  
> >      Before:
> >  
> > -    ```
> > +    ```twig
> @lologhi the standard says that whatever comes after the backticks is the language used by the highlighting. Using jinja is fine, given it allows it to be supported while twig is not. We also use the jinja highlighting in the rST of symfony-doc (well, except that we registered twig as an alias for jinja to make it easier, given that the doc is rendered on a server for which the config can be tweaked)
> 
> —
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