[DOC] Fix comment on SparkPlanGraphEdge

## What changes were proposed in this pull request?

`fromId` is the child, and `toId` is the parent, see line 127 in `buildSparkPlanGraphNode` above.
The edges in Spark UI also go from child to parent.

## How was this patch tested?

Comment change only. Inspected code above. Inspected how the edges in Spark UI look like.

Closes apache#22268 from juliuszsompolski/sparkplangraphedgedoc.

Authored-by: Juliusz Sompolski <[email protected]>
Signed-off-by: Xiao Li <[email protected]>

Author: juliuszsompolski

sql/core/src/main/scala/org/apache/spark/sql/execution/ui/SparkPlanGraph.scala

@@ -202,7 +202,7 @@ private[ui] class SparkPlanGraphCluster(
 
 
 /**
- * Represent an edge in the SparkPlan tree. `fromId` is the parent node id, and `toId` is the child
+ * Represent an edge in the SparkPlan tree. `fromId` is the child node id, and `toId` is the parent
  * node id.
  */
 private[ui] case class SparkPlanGraphEdge(fromId: Long, toId: Long) {