update app.py to have consistent spacing (#3659)

shouldn't have both tabs and spaces in the same file

get-started/part2.md

@@ -126,7 +126,7 @@ def hello():
     return html.format(name=os.getenv("NAME", "world"), hostname=socket.gethostname(), visits=visits)
 
 if __name__ == "__main__":
-	app.run(host='0.0.0.0', port=80)
+    app.run(host='0.0.0.0', port=80)
 ```
 
 Now we see that `pip install -r requirements.txt` installs the Flask and Redis