[DOCS] Use correct event names in changelog

Events are named AfterSectionMarkupGeneratedEvent and
BeforeSectionMarkupGeneratedEvent.

Resolves: #90397
Releases: master
Change-Id: I6a4b690894321d10a618f6056e0790a69d0921d6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63273


Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>