Update KMeansExample.scala

## What changes were proposed in this pull request?
example does not work wo DataFrame import

## How was this patch tested?

example doc only

example does not work wo DataFrame import

Author: Örjan Lundberg <orjan.lundberg@gmail.com>

Closes #12277 from oluies/patch-1.

examples/src/main/scala/org/apache/spark/examples/ml/KMeansExample.scala

@@ -23,8 +23,8 @@ import org.apache.spark.{SparkConf, SparkContext}
 // $example on$
 import org.apache.spark.ml.clustering.KMeans
 import org.apache.spark.mllib.linalg.Vectors
-// $example off$
 import org.apache.spark.sql.{DataFrame, SQLContext}
+// $example off$
 
 /**
  * An example demonstrating a k-means clustering.