fix docs and change minInstancesPerNode to 1

Author: chouqin

mllib/src/main/scala/org/apache/spark/mllib/tree/configuration/Strategy.scala

@@ -49,8 +49,8 @@ import org.apache.spark.mllib.tree.configuration.QuantileStrategy._
  *                                k) implies the feature n is categorical with k categories 0,
  *                                1, 2, ... , k-1. It's important to note that features are
  *                                zero-indexed.
- * @param minInstancesPerNode Minimum number of nodes each child must have after split.
- *                            Default value is 0. If a split cause left or right child
+ * @param minInstancesPerNode Minimum number of instances each child must have after split.
+ *                            Default value is 1. If a split cause left or right child
  *                            to have less than minInstancesPerNode,
  *                            this split will not be considered as a valid split.
  * @param minInfoGain Minimum information gain a split must get. Default value is 0.0.
@@ -68,7 +68,7 @@ class Strategy (
     val maxBins: Int = 32,
     val quantileCalculationStrategy: QuantileStrategy = Sort,
     val categoricalFeaturesInfo: Map[Int, Int] = Map[Int, Int](),
-    val minInstancesPerNode: Int = 0,
+    val minInstancesPerNode: Int = 1,
     val minInfoGain: Double = 0.0,
     val maxMemoryInMB: Int = 256) extends Serializable {