[TASK] Adjust documentation for custom routing enhancer registration

Correct example TYPO3_CONF_VARS configuration for custom route enhancers
registration.

Releases: master, 9.5
Resolves: #88067
Change-Id: I0870498781f496d9c00331af126b96fd77eee70d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60369


Tested-by: Sebastian Rosskopf <sebastian.rosskopf@gmx.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: André Schließer <andy.schliesser@gmail.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Sebastian Rosskopf <sebastian.rosskopf@gmx.de>
Reviewed-by: André Schließer <andy.schliesser@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>