[SPARK-25866][ML] Update KMeans formatVersion

## What changes were proposed in this pull request?

When we added the `distanceMeasure`, we didn't update the `formatVersion` for `KMeans`. Despite this is not a big issue, as that information is used nowhere, we are returning a wrong information.

## How was this patch tested?

NA

Closes apache#22873 from mgaido91/SPARK-25866.

Authored-by: Marco Gaido <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>
(cherry picked from commit 6b42587)
Signed-off-by: Wenchen Fan <[email protected]>

Author: mgaido91

mllib/src/main/scala/org/apache/spark/mllib/clustering/KMeansModel.scala

@@ -113,7 +113,7 @@ class KMeansModel (@Since("1.0.0") val clusterCenters: Array[Vector],
     KMeansModel.SaveLoadV2_0.save(sc, this, path)
   }
 
-  override protected def formatVersion: String = "1.0"
+  override protected def formatVersion: String = "2.0"
 }
 
 @Since("1.4.0")