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

add missing comma and space

Author: Alain <aihe@usc.edu>

Closes #5621 from AiHe/tree-node-issue and squashes the following commits:

159a7bb [Alain] [Minor][MLLIB] Fix a minor formatting bug in toString methods in Node.scala

(cherry picked from commit 4508f01890a723f80d631424ff8eda166a13a727)
Signed-off-by: Xiangrui Meng <meng@databricks.com>

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

@@ -52,7 +52,7 @@ class Node (
 
   override def toString: String = {
     "id = " + id + ", isLeaf = " + isLeaf + ", predict = " + predict + ", " +
-      "impurity =  " + impurity + "split = " + split + ", stats = " + stats
+      "impurity =  " + impurity + ", split = " + split + ", stats = " + stats
   }
 
   /**