KRunner: Improve code snippet

Summary: The code snippet now uses the Qt5 syntax for the signals and the formatting has been fixed.

Test Plan: /

Reviewers: Plasma, davidedmundson, broulik, ngraham

Reviewed By: Plasma, davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28574