Doc: Fix example related to partition pruning

Append node has been removed in v12 when there would be only one subnode
under it.

Author: Amit Langote
Discussion: https://postgr.es/m/CA+HiwqHhS62w8zUFXF4NBjvMboCXYnD-jWoWp-tfo2aHvP3Gxg@mail.gmail.com
Backpatch-through: 12

doc/src/sgml/ddl.sgml

@@ -4525,7 +4525,6 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2008-01-01';
                                     QUERY PLAN
 -----------------------------------------------------------------------------------
  Aggregate  (cost=37.75..37.76 rows=1 width=8)
-   ->  Append  (cost=0.00..36.21 rows=617 width=0)
    ->  Seq Scan on measurement_y2008m01  (cost=0.00..33.12 rows=617 width=0)
          Filter: (logdate >= '2008-01-01'::date)
 </programlisting>