Use default as the db.

Author: yhuai

python/pyspark/sql/context.py

@@ -554,7 +554,7 @@ def tableNames(self, dbName=None):
         >>> sqlContext.registerDataFrameAsTable(df, "table1")
         >>> "table1" in sqlContext.tableNames()
         True
-        >>> "table1" in sqlContext.tableNames("db")
+        >>> "table1" in sqlContext.tableNames("default")
         True
         """
         if dbName is None: