fix download replacement script

scripts/download_backtest_data.py

@@ -1,8 +1,10 @@
+#!/usr/bin/env python3
+
 import sys
 
 
 print("This script has been integrated into freqtrade "
-      "and it's functionality is available by calling `freqtrade download-data`.")
+      "and its functionality is available by calling `freqtrade download-data`.")
 print("Please check the documentation on https://www.freqtrade.io/en/latest/backtesting/ "
       "for details.")