[KMessageWidget] Subtract the correct margin
Summary: The comment describes a 1px border but the border actually has a size of 2px.
Test Plan:
Before:
After:
Reviewers: Frameworks, SGOrava, ngraham
Reviewed By: ngraham
Subscribers: ngraham, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D26767