Fix "wider column" width in the grid sample

The changes of the commit 393f5c8e2b (Use less arbitrary column/row
sizes in the grid sample, 2020-03-31) accidentally made this column
narrower than default instead of making it wider due to a typo in the
name of wxGrid method: it should be the default column size, not the
default column label size.

samples/grid/griddemo.cpp

@@ -534,7 +534,7 @@ GridFrame::GridFrame()
     grid->SetRowAttr(5, attr);
 
     grid->SetCellValue(2, 4, "a wider column");
-    grid->SetColSize(4, 3*grid->GetDefaultColLabelSize()/2);
+    grid->SetColSize(4, 3*grid->GetDefaultColSize()/2);
     grid->SetColMinimalWidth(4, grid->GetColSize(4));
 
     grid->SetCellTextColour(5, 8, *wxGREEN);