Doc: fix example of recursive query.

Compute total number of sub-parts correctly, per jason@banfelder.net

Simon Riggs

Discussion: https://postgr.es/m/166161184718.1235920.6304070286124217754@wrigleys.postgresql.org

doc/src/sgml/queries.sgml

@@ -2133,7 +2133,7 @@ SELECT sum(n) FROM t;
 WITH RECURSIVE included_parts(sub_part, part, quantity) AS (
     SELECT sub_part, part, quantity FROM parts WHERE part = 'our_product'
   UNION ALL
-    SELECT p.sub_part, p.part, p.quantity
+    SELECT p.sub_part, p.part, p.quantity * pr.quantity
     FROM included_parts pr, parts p
     WHERE p.part = pr.sub_part
 )