minor change on format

Author: zjffdu

python/pyspark/sql/column.py

@@ -270,7 +270,7 @@ def __iter__(self):
 
     @ignore_unicode_prefix
     @since(1.3)
-    def substr(self, startPos, length = None):
+    def substr(self, startPos, length=None):
         """
         Return a :class:`Column` which is a substring of the column.