Fix a missing article in the documentation

Per complaint from Grant Gryczan.

It's a very old typo; backpatch all the way back.

Author: Laurenz Albe <laurenz.albe@cybertec.at>
Discussion: https://postgr.es/m/172179789219.915368.16590585529628354757@wrigleys.postgresql.org

Author: alvherre

doc/src/sgml/parallel.sgml

@@ -202,7 +202,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
   </itemizedlist>
 
   <para>
-    Even when parallel query plan is generated for a particular query, there
+    Even when a parallel query plan is generated for a particular query, there
     are several circumstances under which it will be impossible to execute
     that plan in parallel at execution time.  If this occurs, the leader
     will execute the portion of the plan below the <literal>Gather</literal>