Increase backend header text's left margin

Summary:
Give the backend name a bit more room so it doesn't look so cramped on the left side. Even though this makes the left edge of the text no longer align with the checkboxes, somehow it looks better overall to me.

CCBUG: 396833

Test Plan:
Before:

After:

Reviewers: apol, Discover Software Store, VDG

Reviewed By: apol, Discover Software Store

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14385