Fix default fixed font in fonts kcm

Summary:
Now it is the same as used by plasma-integration.
Also adjust the file name in the comment, which was apparently forgotten.

Test Plan: Hitting "Defaults" in kcmshell5 fonts now shows "Hack 9" instead of "Oxygen Mono 9".

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4794