[MINOR][DOCS] Remove remaining sqlContext in documentation at examples

This PR removes `sqlContext` in examples. Actual usage was all replaced in https://github.com/apache/spark/pull/12809 but there are some in comments.

Manual style checking.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #13006 from HyukjinKwon/minor-docs.

examples/src/main/java/org/apache/spark/examples/sql/JavaSparkSQL.java

@@ -133,7 +133,7 @@ public class JavaSparkSQL {
     // Register this DataFrame as a table.
     peopleFromJsonFile.registerTempTable("people");
 
-    // SQL statements can be run by using the sql methods provided by sqlContext.
+    // SQL statements can be run by using the sql methods provided by `spark`
     Dataset<Row> teenagers3 = spark.sql("SELECT name FROM people WHERE age >= 13 AND age <= 19");
 
     // The results of SQL queries are DataFrame and support all the normal RDD operations.

examples/src/main/python/sql.py

@@ -69,7 +69,7 @@ if __name__ == "__main__":
     # Register this DataFrame as a temporary table.
     people.registerTempTable("people")
 
-    # SQL statements can be run by using the sql methods provided by sqlContext
+    # SQL statements can be run by using the sql methods provided by `spark`
     teenagers = spark.sql("SELECT name FROM people WHERE age >= 13 AND age <= 19")
 
     for each in teenagers.collect():