Remove bug from OPTIMIZER_DEBUG code for partition-wise join.

Etsuro Fujita, reviewed by Ashutosh Bapat

Discussion: http://postgr.es/m/5A2A60E6.6000008@lab.ntt.co.jp

src/backend/optimizer/path/allpaths.c

@@ -3457,7 +3457,7 @@ generate_partition_wise_join_paths(PlannerInfo *root, RelOptInfo *rel)
 		set_cheapest(child_rel);
 
 #ifdef OPTIMIZER_DEBUG
-		debug_print_rel(root, rel);
+		debug_print_rel(root, child_rel);
 #endif
 
 		live_children = lappend(live_children, child_rel);