Use i18np for a plural string

Summary:
Use i18np() even though this string will always be plural in English; other languages are different, so we should use the correct i18np() call.

BUG: 395951

Reviewers: Discover Software Store, apol

Reviewed By: Discover Software Store, apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D13785