Fix positioning of bottom-aligned checkboxes in wxGrid

This fixes a bug due to a typo in ab02d36e10 (Account for vertical
alignment in wxGetGridCheckBoxRect() too, 2019-11-22) which used the
vertical component of a wrong object and so didn't actually offset the
checkbox by its size from the bottom of the cell rectangle, as it should
have done.

Closes #18907.

src/generic/grid.cpp

@@ -11240,7 +11240,7 @@ wxGetContentRect(wxSize contentSize,
     else if ( vAlign & wxALIGN_BOTTOM )
     {
         contentRect.SetY(cellRect.y + cellRect.height
-                          - contentRect.y - GRID_CELL_CHECKBOX_MARGIN);
+                          - contentSize.y - GRID_CELL_CHECKBOX_MARGIN);
     }
     else // wxALIGN_TOP
     {