Large cactus schematic: Don't force-place root node

This schematic is used for sapling-grown large cacti, so should not
destroy the sand node below, as sand nodes are in short supply in
some situations.
Like trees, only force-place the node that replaces the sapling.

schematic_tables.txt

@@ -2068,7 +2068,7 @@ mts_save("large_cactus", {
 		_, _, _, _, _,
 		_, _, _, _, _,
 
-		_, _, R, _, _,
+		_, _, C, _, _,
 		_, _, R, _, _,
 		_, _, C, _, _,
 		C, C, C, C, C,