fixed typo on README of xss vulnerability scanner tutorial

Author: x4nth055

ethical-hacking/xss-vulnerability-scanner/README.md

@@ -3,5 +3,5 @@ To run this:
 - `pip3 install -r requirements.txt`
 - To detect XSS on `https://www.example.com`:
     ```
-    python xss_scanner https://www.example.com
+    python xss_scanner.py https://www.example.com
     ```