Fix warning about hiding a virtual function in propgrid sample.

The virtual ColourToString() method was hidden and not overridden in the
sample because it didn't use the last argument of the base class method.

Fix this in the most straightforward (albeit not necessarily the most correct)
way to at least ensure that the function is really overridden.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68109 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775