[MINOR][DOCS] Typo fixes in Dataset scaladoc

## What changes were proposed in this pull request?

Minor typo fixes in Dataset scaladoc
* Corrected context type as SparkSession, not SQLContext.
liancheng rxin andrewor14

## How was this patch tested?

Compiled locally

Author: Xinh Huynh <xinh_huynh@yahoo.com>

Closes #13330 from xinhhuynh/fix-dataset-typos.

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

@@ -121,7 +121,7 @@ private[sql] object Dataset {
  *
  * A more concrete example in Scala:
  * {{{
- *   // To create Dataset[Row] using SQLContext
+ *   // To create Dataset[Row] using SparkSession
  *   val people = spark.read.parquet("...")
  *   val department = spark.read.parquet("...")
  *
@@ -133,7 +133,7 @@ private[sql] object Dataset {
  *
  * and in Java:
  * {{{
- *   // To create Dataset<Row> using SQLContext
+ *   // To create Dataset<Row> using SparkSession
  *   Dataset<Row> people = spark.read().parquet("...");
  *   Dataset<Row> department = spark.read().parquet("...");
  *