[PySpark][Minor] Update sql example, so that can read file correctly

To run Spark, default will read file from HDFS if we don't set the schema.

Author: linweizhong <linweizhong@huawei.com>

Closes #5684 from Sephiroth-Lin/pyspark_example_minor and squashes the following commits:

19fe145 [linweizhong] Update example sql.py, so that can read file correctly

examples/src/main/python/sql.py

@@ -18,6 +18,7 @@
 from __future__ import print_function
 
 import os
+import sys
 
 from pyspark import SparkContext
 from pyspark.sql import SQLContext
@@ -50,7 +51,11 @@ if __name__ == "__main__":
 
     # A JSON dataset is pointed to by path.
     # The path can be either a single text file or a directory storing text files.
-    path = os.path.join(os.environ['SPARK_HOME'], "examples/src/main/resources/people.json")
+    if len(sys.argv) < 2:
+        path = "file://" + \
+            os.path.join(os.environ['SPARK_HOME'], "examples/src/main/resources/people.json")
+    else:
+        path = sys.argv[1]
     # Create a DataFrame from the file(s) pointed to by path
     people = sqlContext.jsonFile(path)
     # root