[MINOR][MLLIB] move setCheckpointInterval to non-expert setters

## What changes were proposed in this pull request?

The `checkpointInterval` is a non-expert param. This PR moves its setter to non-expert group.

Author: Xiangrui Meng <meng@databricks.com>

Closes #13813 from mengxr/checkpoint-non-expert.

mllib/src/main/scala/org/apache/spark/ml/tree/treeParams.scala

@@ -151,7 +151,7 @@ private[ml] trait DecisionTreeParams extends PredictorParams
    * [[org.apache.spark.SparkContext]].
    * Must be >= 1.
    * (default = 10)
-   * @group expertSetParam
+   * @group setParam
    */
   def setCheckpointInterval(value: Int): this.type = set(checkpointInterval, value)