[DOCS] Correct code-block directive

The code-block directive needs two colons to be rendered
correctly.

Resolves: #89922
Related: #89463
Releases: master
Change-Id: Ida99628710cc3558a0dbdc39272e788c390ca154
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62600


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>