Update heroku.md

[manaswii]

flask login was having many issues for me, it kept logging in and out, sessions were inconsistent.
This fixed it for me,
source-https://stackoverflow.com/questions/39684364/heroku-gunicorn-flask-login-is-not-working-properly?rq=1

[dmalan]

@manaswii, do you have precisely the below lines in your app.py or something different?

app.config["SESSION_PERMANENT"] = False
app.config["SESSION_TYPE"] = "filesystem"
Session(app)

[manaswii]

@dmalan
This is the exact excerpt from my project

app.config["SESSION_FILE_DIR"] = mkdtemp()
app.config["SESSION_PERMANENT"] = False
app.config["SESSION_TYPE"] = "filesystem"
Session(app)