[DOCS] Properly document modulename registration for EXT:adminpanel

The documentation contained a wrong registration code example,
which now uses the proper array depth level notation.

Releases: main, 12.4, 11.5
Resolves: #104105
Change-Id: Idb3c9b0e723af2f1bb579bc304637497e1272e36
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84645


Tested-by: core-ci <typo3@b13.com>
Tested-by: Lina Wolf <112@linawolf.de>
Reviewed-by: Lina Wolf <112@linawolf.de>