Correct KFormat::formatBytes examples

Summary:
The example left out the "precision" parameter, reported values where
wrong (values are rounded, not truncated). The precision parameter is
ignored for values less than 1000 resp. 1024.

Test Plan: make test

Reviewers: Frameworks, dhaumann

Reviewed By: dhaumann

Subscribers: dhaumann, apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13571