Fix MAIL_SUPPRESS_SEND documentation bug

I think the docs for MAIL_SUPPRESS_SEND setting has a bug. It says:
> Alternatively outside a testing environment you can set MAIL_SUPPRESS_SEND to **False**. This will have the same effect.

I think this happens when MAIL_SUPPRESS_SEND is set to **True**. At least that's what I could understand by reading the setting's name. Also, by looking at the code on [`Connection.__enter__`][1] method, it looks like this:

    if self.mail.suppress:
        self.host = None
    else:
        self.host = self.configure_host()

Meaning that when `self.mail.suppress == True` then the message is not gonna be delivered. At least this is what I understand. Please correct me if I'm wrong. Either way here is the very small patch to the doc in case I'm right.

  [1]:https://github.com/mattupstate/flask-mail/blob/master/flask_mail.py#L140