Fix PageRow globalToolBar documentation

Summary: In PageRow documentation, globalToolBar's style contains an unordered list of the various style value options. With this commit, the list is properly displayed.

Reviewers: ognarb, ahiemstra

Reviewed By: ognarb, ahiemstra

Subscribers: ahiemstra, ognarb, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D25104