merge spark.ml and spark.mllib into the same group in javadoc

Author: mengxr

project/SparkBuild.scala

@@ -367,9 +367,7 @@ object Unidoc {
         "mllib.regression", "mllib.stat", "mllib.tree", "mllib.tree.configuration",
         "mllib.tree.impurity", "mllib.tree.model", "mllib.util",
         "mllib.evaluation", "mllib.feature", "mllib.random", "mllib.stat.correlation",
-        "mllib.stat.test", "mllib.tree.impl", "mllib.tree.loss"
-      ),
-      "-group", "ML", packageList(
+        "mllib.stat.test", "mllib.tree.impl", "mllib.tree.loss",
         "ml", "ml.classification", "ml.evaluation", "ml.feature", "ml.param", "ml.tuning"
       ),
       "-group", "Spark SQL", packageList("sql.api.java", "sql.api.java.types", "sql.hive.api.java"),