[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>