[MINOR][DOCS] Correct the usage example of Dataset in Java

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Closes #44342 from Aiden-Dong/aiden-dev.

Lead-authored-by: aiden <dongtao@haier.com>
Co-authored-by: Hyukjin Kwon <gurwls223@gmail.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala

@@ -138,7 +138,8 @@ private[sql] object Dataset {
  * the following creates a new Dataset by applying a filter on the existing one:
  * {{{
  *   val names = people.map(_.name)  // in Scala; names is a Dataset[String]
- *   Dataset<String> names = people.map((Person p) -> p.name, Encoders.STRING));
+ *   Dataset<String> names = people.map(
+ *     (MapFunction<Person, String>) p -> p.name, Encoders.STRING()); // Java
  * }}}
  *
  * Dataset operations can also be untyped, through various domain-specific-language (DSL)