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