Fix bogus comment.

Commit 4212cb7 rendered a comment
in execMain.c incorrect.  Per complaint from Tom Lane, repair.

Patch from Amit Kapila, per wording suggested by Tom Lane and me.

Author: robertmhaas

src/backend/executor/execMain.c

@@ -1540,9 +1540,10 @@ ExecutePlan(EState *estate,
 	estate->es_direction = direction;
 
 	/*
-	 * If a tuple count was supplied or data is being written to relation, we
-	 * must force the plan to run without parallelism, because we might exit
-	 * early.
+	 * If a tuple count was supplied, we must force the plan to run without
+	 * parallelism, because we might exit early.  Also disable parallelism
+	 * when writing into a relation, because no database changes are allowed
+	 * in parallel mode.
 	 */
 	if (numberTuples || dest->mydest == DestIntoRel)
 		use_parallel_mode = false;