Fix broken example in PL/pgSQL document.

Back-patch to all supported branches.

Marti Raudsepp, per a report from Marko Tiikkaja

doc/src/sgml/plpgsql.sgml

@@ -487,8 +487,8 @@ $$ LANGUAGE plpgsql;
 CREATE FUNCTION extended_sales(p_itemno int)
 RETURNS TABLE(quantity int, total numeric) AS $$
 BEGIN
-    RETURN QUERY SELECT quantity, quantity * price FROM sales
-                 WHERE itemno = p_itemno;
+    RETURN QUERY SELECT s.quantity, s.quantity * s.price FROM sales AS s
+                 WHERE s.itemno = p_itemno;
 END;
 $$ LANGUAGE plpgsql;
 </programlisting>