[DOCS] Correctly state "bigint signed" as datetime default

The original patch in [1] wrongly stated that the column
type was changed to "bigint unsigned", when in fact it
is "bigint signed" (to allow dates before 1970).

This documentation mistake is now fixed.

[1] https://review.typo3.org/c/Packages/TYPO3.CMS/+/80668

Resolves: #103113
Releases: main
Change-Id: Iff9b1cc00a040c4315ce3ee13bb6bd93afb97fb0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82992


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: core-ci <typo3@b13.com>