KFormat: correct @since tag for new KFormat::formatValue

Summary:
The change landed immediately after the KF 5.48 release, so update it to
@since 5.49.

Test Plan: grep 5.48 no longer returns any match

Reviewers: Frameworks, kossebau

Reviewed By: kossebau

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14110