Replace main with __main__ (#3518)

web_programming/slack_message.py

@@ -13,7 +13,7 @@ def send_slack_message(message_body: str, slack_url: str) -> None:
         )
 
 
-if __name__ == "main":
+if __name__ == "__main__":
     # Set the slack url to the one provided by Slack when you create the webhook at
     # https://my.slack.com/services/new/incoming-webhook/
     send_slack_message("<YOUR MESSAGE BODY>", "<SLACK CHANNEL URL>")