[BUGFIX] Fix tree example in styleguide

Since the node is `collapsed` on initial
rendering, the `aria-expanded` attribute
is set to "false".

Resolves: #103119
Releases: main
Change-Id: Ia06d0a268cf572453b17f5bd332e7d482019b561
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82997


Tested-by: Jochen Roth <rothjochen@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Jochen Roth <rothjochen@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>