Simplify applet keyboard shortcut chooser's explanatory text

Summary:
The old test was unnecessarily wordy and contained grammar errors. This patch simplifies
the description using plainer and more comprehensible language.

Test Plan:

Reviewers: VDG, Plasma, ndavis

Reviewed By: VDG, ndavis

Subscribers: ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20997