[kio-extras] Use "storage media" instead of "storages"

Summary: This patch improves the label's grammar.

Reviewers: #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D19053