Fix typo in row height definition

Using last cell's position works too, but it's much cleaner to use row's
position.

Fix #983.

weasyprint/layout/tables.py

@@ -182,7 +182,7 @@ def group_layout(group, position_y, max_position_y,
                 else:
                     row.height = max(row.height, max(
                         row_cell.height for row_cell in ending_cells))
-                    row_bottom_y = cell.position_y + row.height
+                    row_bottom_y = row.position_y + row.height
             else:
                 row_bottom_y = row.position_y
                 row.height = 0