[Minor][MLLIB] Fix a formatting bug in toString method in Node

Modify Predict.scala and decouple the dependency of Node.scala on
Predict.scala

Author: Alain

mllib/src/main/scala/org/apache/spark/mllib/tree/model/Node.scala

@@ -51,7 +51,7 @@ class Node (
     var stats: Option[InformationGainStats]) extends Serializable with Logging {
 
   override def toString: String = {
-    "id = " + id + ", isLeaf = " + isLeaf + ", " + predict + ", " +
+    "id = " + id + ", isLeaf = " + isLeaf + ", predict = " + predict + ", " +
       "impurity = " + impurity + ", split = " + split + ", stats = " + stats
   }
 

mllib/src/main/scala/org/apache/spark/mllib/tree/model/Predict.scala

@@ -29,9 +29,7 @@ class Predict(
     val predict: Double,
     val prob: Double = 0.0) extends Serializable {
 
-  override def toString: String = {
-    "predict = %f, prob = %f".format(predict, prob)
-  }
+  override def toString: String = s"$predict(prob = $prob)"
 
   override def equals(other: Any): Boolean = {
     other match {