[DOCS] Correct example for defining RTE preset of flexform field

When addressing flexform fields in TSconfig, the dots of the field names
have to be escaped with a backslash.

Releases: main, 12.4, 11.5
Resolves: #103786
Change-Id: I6af70fcb80e9e155ba8259ec216c5199600c5a78
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84199


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>