[SPARK-15398][ML] Update the warning message to recommend ML usage

## What changes were proposed in this pull request?
MLlib are not recommended to use, and some methods are even deprecated.
Update the warning message to recommend ML usage.
```
  def showWarning() {
    System.err.println(
      """WARN: This is a naive implementation of Logistic Regression and is given as an example!
        |Please use either org.apache.spark.mllib.classification.LogisticRegressionWithSGD or
        |org.apache.spark.mllib.classification.LogisticRegressionWithLBFGS
        |for more conventional use.
      """.stripMargin)
  }
```
To
```
  def showWarning() {
    System.err.println(
      """WARN: This is a naive implementation of Logistic Regression and is given as an example!
        |Please use org.apache.spark.ml.classification.LogisticRegression
        |for more conventional use.
      """.stripMargin)
  }
```

## How was this patch tested?
local build

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #13190 from zhengruifeng/update_recd.

1 files changed, 2 insertions, 4 deletions

diff --git a/examples/src/main/java/org/apache/spark/examples/JavaHdfsLR.java b/examples/src/main/java/org/apache/spark/examples/JavaHdfsLR.java
index 31a79ddd3f..f64155ce3c 100644
--- a/examples/src/main/java/org/apache/spark/examples/JavaHdfsLR.java
+++ b/examples/src/main/java/org/apache/spark/examples/JavaHdfsLR.java
@@ -32,8 +32,7 @@ import java.util.regex.Pattern;
  * Logistic regression based classification.
  *
  * This is an example implementation for learning how to use Spark. For more conventional use,
- * please refer to either org.apache.spark.mllib.classification.LogisticRegressionWithSGD or
- * org.apache.spark.mllib.classification.LogisticRegressionWithLBFGS based on your needs.
+ * please refer to org.apache.spark.ml.classification.LogisticRegression.
  */
 public final class JavaHdfsLR {
 
@@ -43,8 +42,7 @@ public final class JavaHdfsLR {
   static void showWarning() {
     String warning = "WARN: This is a naive implementation of Logistic Regression " +
             "and is given as an example!\n" +
-            "Please use either org.apache.spark.mllib.classification.LogisticRegressionWithSGD " +
-            "or org.apache.spark.mllib.classification.LogisticRegressionWithLBFGS " +
+            "Please use org.apache.spark.ml.classification.LogisticRegression " +
             "for more conventional use.";
     System.err.println(warning);
   }