[TASK] Fix syntax error in RST file

As some might copy and paste the configuration structure, there should be
no syntax error.
Therefore the missing comma was added.

Releases: master, 8.7
Resolves: #81879
Change-Id: Icaa718fd29ffd52ea1cf9f858ed61b599cfc0ba9
Reviewed-on: https://review.typo3.org/53512


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: TYPO3com <no-reply@typo3.com>