[DOCS] Fix mistakes regarding RTE-configuration via TsConfig

1. "Override for one table" is not implemented in core;
2. Preset "normal" is confusing / non-existing in core. Use
"myCustomPreset" to demonstrate usage of custom presets.
3. Use the same syntax (with square brackets) like in:
https://docs.typo3.org/m/typo3/reference-tsconfig/master/en-us/PageTsconfig/Rte.html

Releases: master, 9.5
Resolves: #89696
Change-Id: I45c51d6afd4a1f46fc377715c24952f781858d54
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62334


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Daniel Siepmann <coding@daniel-siepmann.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Daniel Siepmann <coding@daniel-siepmann.de>