[BUGFIX] Code block in docs regarding site base variants

Do not use inline notion of providing a whole block of code.

Relates: #88630
Releases: master, 9.5
Change-Id: I2f691cbdd4d617bf1a0ac4a82000f809f865c069
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61169


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