[kcm trash] Change kcm trash size percent to 2 decimal places

Summary: This patch changes the precision from 3 decimal places to 2

Test Plan: Go to dolphin->Settings->Configure Dolhin->Trash

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29513