[SPARK-20694][EXAMPLES] Update SQLDataSourceExample.scala

## What changes were proposed in this pull request?

Create table using the right DataFrame. peopleDF->usersDF

peopleDF:
+----+-------+
| age|   name|
+----+-------+
usersDF:
+------+--------------+----------------+
|  name|favorite_color|favorite_numbers|
+------+--------------+----------------+

## How was this patch tested?

Manually tested.

Author: CNRui <13266776177@163.com>

Closes #20052 from CNRui/patch-2.

examples/src/main/scala/org/apache/spark/examples/sql/SQLDataSourceExample.scala

@@ -67,15 +67,15 @@ object SQLDataSourceExample {
     usersDF.write.partitionBy("favorite_color").format("parquet").save("namesPartByColor.parquet")
     // $example off:write_partitioning$
     // $example on:write_partition_and_bucket$
-    peopleDF
+    usersDF
       .write
       .partitionBy("favorite_color")
       .bucketBy(42, "name")
-      .saveAsTable("people_partitioned_bucketed")
+      .saveAsTable("users_partitioned_bucketed")
     // $example off:write_partition_and_bucket$
 
     spark.sql("DROP TABLE IF EXISTS people_bucketed")
-    spark.sql("DROP TABLE IF EXISTS people_partitioned_bucketed")
+    spark.sql("DROP TABLE IF EXISTS users_partitioned_bucketed")
   }
 
   private def runBasicParquetExample(spark: SparkSession): Unit = {