[DOCS] Fix broken link in fluid_styled_content docs

Fixed a broken link which points to the "register extension key" page.

Releases: master, 9.5
Resolves: #89642
Change-Id: I5a5408b0690a0d3949cf27136ead1223a12a0304
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62297


Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Sybille Peters <sypets@gmx.de>
Reviewed-by: Daniel Siepmann <coding@daniel-siepmann.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>