[SPARK-27815][SQL][FOLLOWUP][DOC] Update comment that references `Pus…

…hDownPredicate`

## What changes were proposed in this pull request?

The optimize rule `PushDownPredicate` has been combined into `PushDownPredicates`, update the comment that references the old rule.

## How was this patch tested?

N/A

Closes apache#25207 from jiangxb1987/comment.

Authored-by: Xingbo Jiang <xingbo.jiang@databricks.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -487,7 +487,7 @@ object LimitPushDown extends Rule[LogicalPlan] {
  * Union:
  * Right now, Union means UNION ALL, which does not de-duplicate rows. So, it is
  * safe to pushdown Filters and Projections through it. Filter pushdown is handled by another
- * rule PushDownPredicate. Once we add UNION DISTINCT, we will not be able to pushdown Projections.
+ * rule PushDownPredicates. Once we add UNION DISTINCT, we will not be able to pushdown Projections.
  */
 object PushProjectionThroughUnion extends Rule[LogicalPlan] with PredicateHelper {