Port endl to "\n". endl in qt5.15 is namespaced. We don't need to flush as when QFile is deleted it flush data

Summary: Port endl to \n

Reviewers: dfaure, aacid

Reviewed By: aacid

Subscribers: ltoscano, aacid, anthonyfieroni, kde-frameworks-devel, kde-doc-english

Tags: Frameworks, Documentation

Differential Revision: https://phabricator.kde.org/D26317