Fix format specifier in an error message in dataview sample

Use "%zu" for a size_t value to avoid assert failures under LP64 (i.e.
all 64-bit Unix) systems.

samples/dataview/mymodels.cpp

@@ -591,7 +591,7 @@ bool MyListModel::SetValueByRow( const wxVariant &variant,
             {
                 // the item is not in the range of the items
                 // which we store... for simplicity, don't allow editing it
-                wxLogError( "Cannot edit rows with an index greater than %d",
+                wxLogError( "Cannot edit rows with an index greater than %zu",
                             m_textColValues.GetCount() );
                 return false;
             }