[MINOR][DOC] Fix doc of ForeachWriter to use writeStream

## What changes were proposed in this pull request?

Fix the document of `ForeachWriter` to use `writeStream` instead of `write` for a streaming dataset.

## How was this patch tested?
Docs only.

Author: Carson Wang <carson.wang@intel.com>

Closes #16419 from carsonwang/FixDoc.

(cherry picked from commit 2a5f52a7)
Signed-off-by: Sean Owen <sowen@cloudera.com>

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

@@ -28,7 +28,7 @@ import org.apache.spark.annotation.{Experimental, InterfaceStability}
  *
  * Scala example:
  * {{{
- *   datasetOfString.write.foreach(new ForeachWriter[String] {
+ *   datasetOfString.writeStream.foreach(new ForeachWriter[String] {
  *
  *     def open(partitionId: Long, version: Long): Boolean = {
  *       // open connection
@@ -46,7 +46,7 @@ import org.apache.spark.annotation.{Experimental, InterfaceStability}
  *
  * Java example:
  * {{{
- *  datasetOfString.write().foreach(new ForeachWriter<String>() {
+ *  datasetOfString.writeStream().foreach(new ForeachWriter<String>() {
  *
  *    @Override
  *    public boolean open(long partitionId, long version) {