[BUGFIX] Use correct date format in EXT:styleguide date picker example

The date picker used in EXT:styleguide -> Components -> Form needs
another format to be functional again.

Resolves: #104660
Releases: main
Change-Id: Iec506bbdbb68d3f41c6e9c0fb5701fc5ba0522d3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85678


Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>