[DOCS] Fix PHP syntax in PageContentPreviewRenderingEvent example

The $event parameter is now added and a superfluous parenthesis is
removed in the condition. Also, the $tableName variable is used now.

Resolves: #103645
Related: #98375
Releases: main, 12.4
Change-Id: I8f827ac7f6941ab03a6f5c67e9fb3525d1a79868
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83843


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Bartsch <bo@cedev.de>