Fix identation

Author: rnowling

mllib/src/main/scala/org/apache/spark/mllib/feature/IDF.scala

@@ -78,7 +78,7 @@ private object IDF {
     private var df: BDV[Long] = _
 
 
-   def this() = this(0)
+    def this() = this(0)
 
     /** Adds a new document. */
     def add(doc: Vector): this.type = {