Update wxListEvent::GetData() documentation too.

It returns wxUIntPtr and not long since r75000, see #15578.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75135 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775