KFormat: fix typo in SI prefix name enum

Summary:
Its milli, not mili. Change the enum name without any backwards
compatibility, as the name is not yet in any release.

Reviewers: Frameworks, astippich, pino

Reviewed By: pino

Subscribers: pino, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14135