GTK theme treeview style typo/bug fix

Summary:
Fixes a typo in the CSS files which resulted in GTK treeviews being unusable. See picture.
{F6339414}

Reviewers: #breeze, #vdg, broulik, ngraham

Reviewed By: #breeze, #vdg, broulik, ngraham

Subscribers: ngraham, jackg, plasma-devel, #breeze

Tags: #plasma, #breeze

Differential Revision: https://phabricator.kde.org/D16331