Adjust label in the settings page for changed preview behaviour

Summary: Requested by https://phabricator.kde.org/D15138#318904

Test Plan: Opened the preview settings, label text changed.

Reviewers: #dolphin, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: ngraham, markg, broulik, kfm-devel, elvisangelaccio

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D15216