fix copy paste in assertion message (vcl/gtk3/gtkinst)

Change-Id: I2603dcb809d2bf062f3b14f7c21583fde1da9f05
diff --git a/vcl/unx/gtk3/gtkinst.cxx b/vcl/unx/gtk3/gtkinst.cxx
index 053bf5a0..5a42708 100644
--- a/vcl/unx/gtk3/gtkinst.cxx
+++ b/vcl/unx/gtk3/gtkinst.cxx
@@ -16985,7 +16985,7 @@ public:
    {
        //get_accessible_relation_set should only be needed for the vcl implementation,
        //in the gtk impl the native equivalent should negate the need.
        assert(false && "get_accessible_parent should only be called on a vcl impl");
        assert(false && "get_accessible_relation_set should only be called on a vcl impl");
        return uno::Reference<css::accessibility::XAccessibleRelationSet>();
    }