[MINOR] [PYSPARK] [EXAMPLES] Changed examples to use SparkSession.sparkContext instead of _sc

## What changes were proposed in this pull request?

Some PySpark examples need a SparkContext and get it by accessing _sc directly from the session.  These examples should use the provided property `sparkContext` in `SparkSession` instead.

## How was this patch tested?
Ran modified examples

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #13303 from BryanCutler/pyspark-session-sparkContext-MINOR.

examples/src/main/python/als.py

@@ -67,7 +67,7 @@ if __name__ == "__main__":
         .appName("PythonALS")\
         .getOrCreate()
 
-    sc = spark._sc
+    sc = spark.sparkContext
 
     M = int(sys.argv[1]) if len(sys.argv) > 1 else 100
     U = int(sys.argv[2]) if len(sys.argv) > 2 else 500

examples/src/main/python/avro_inputformat.py

@@ -70,7 +70,7 @@ if __name__ == "__main__":
         .appName("AvroKeyInputFormat")\
         .getOrCreate()
 
-    sc = spark._sc
+    sc = spark.sparkContext
 
     conf = None
     if len(sys.argv) == 3:

examples/src/main/python/parquet_inputformat.py

@@ -53,7 +53,7 @@ if __name__ == "__main__":
         .appName("ParquetInputFormat")\
         .getOrCreate()
 
-    sc = spark._sc
+    sc = spark.sparkContext
 
     parquet_rdd = sc.newAPIHadoopFile(
         path,

examples/src/main/python/pi.py

@@ -32,7 +32,7 @@ if __name__ == "__main__":
         .appName("PythonPi")\
         .getOrCreate()
 
-    sc = spark._sc
+    sc = spark.sparkContext
 
     partitions = int(sys.argv[1]) if len(sys.argv) > 1 else 2
     n = 100000 * partitions

examples/src/main/python/transitive_closure.py

@@ -46,7 +46,7 @@ if __name__ == "__main__":
         .appName("PythonTransitiveClosure")\
         .getOrCreate()
 
-    sc = spark._sc
+    sc = spark.sparkContext
 
     partitions = int(sys.argv[1]) if len(sys.argv) > 1 else 2
     tc = sc.parallelize(generateGraph(), partitions).cache()