Add plus symbol to document-new.svg

Summary:
BUG: 398850
FIXED-IN: 5.51

Test Plan: Works for me

Reviewers: #breeze, #vdg, ngraham

Reviewed By: #breeze, #vdg, ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D15618