[DOCS] Update placeholder spelling in screenshot of EXT:reactions docs

"{text}/{header}" needs to be replaced with
"${text}/${header} " for proper substitution.

Resolves: #105358
Releases: main, 13.4, 12.4
Change-Id: Ia0538f334ad8438ca3a5e405efbc4f54e66851fc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/87486
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Garvin Hicking <gh@faktor-e.de>

Author: minapok