demo: Don't use localizeddate to avoid Intl errors

I didn’t have a supported timezone setting in my PHP.ini, resulting in the following exception:

An exception has been thrown during the rendering of a template (“Time zone different than GMT or UTC is not supported as a formatting output. Please install the “intl” extension for full localization capabilities.”) in base.html.twig at line 150.

I think this will be a common issue for Windows users. As this demo should work straightaway, I propose to remove |localizeddate from the templates and simply replace them by |date('r') or the like.