Address comment.

Author: viirya

mllib/src/main/scala/org/apache/spark/ml/stat/Correlation.scala

@@ -62,8 +62,8 @@ object Correlation {
    *
    * @note For Spearman, a rank correlation, we need to create an RDD[Double] for each column
    * and sort it in order to retrieve the ranks and then join the columns back into an RDD[Vector],
-   * which is fairly costly. Cache the input RDD before calling corr with `method = "spearman"` to
-   * avoid recomputing the common lineage.
+   * which is fairly costly. Cache the input Dataset before calling corr with `method = "spearman"`
+   * to avoid recomputing the common lineage.
    */
   @Since("2.2.0")
   def corr(dataset: Dataset[_], column: String, method: String): DataFrame = {