x11/gtk-app-menu: add WWW to pkg-descr

The gtk-app-menu library is a sub-project for vala-panel-appmenu,
add an appropriate WWW line in pkg-descr

Maybe someday we can add the other panels, but as of now, we
are just using this sub-project.

PR: 254356
Submitted by: Trenton Schulz <trueos@norwegianrockcat.com>
Reported by: Graham Perrin <grahamperrin@gmail.com>