[SQL] Mark strategies with override for clarity.

## What changes were proposed in this pull request?

This is a very trivial PR, simply marking `strategies` in `SparkPlanner` with the `override` keyword for clarity since it is overriding `strategies` in `QueryPlanner` two levels up in the class hierarchy. I was reading through the code to learn a bit and got stuck on this fact for a little while, so I figured this may be helpful so that another developer new to the project doesn't get stuck where I was.

I did not make a JIRA ticket for this because it is so trivial, but I'm happy to do so to adhere to the contribution guidelines if required.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Eric Perry <eric@ericjperry.com>

Closes #19537 from ericjperry/override-strategies.

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlanner.scala

@@ -33,7 +33,7 @@ class SparkPlanner(
 
   def numPartitions: Int = conf.numShufflePartitions
 
-  def strategies: Seq[Strategy] =
+  override def strategies: Seq[Strategy] =
     experimentalMethods.extraStrategies ++
       extraPlanningStrategies ++ (
       DataSourceV2Strategy ::