Refine tab bar settings UI string

Summary:
Make the Qt 5.9 sentence present tense since we already
passed that version, and add a minor grammar update.

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: ngraham, hindenburg, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D13125