Fix type spec of Mailer.deliver_later/1

[lasseebert]

After updating to 2.0.0, my dialyzer fails with a message like this:

lib/my_app/mailer/mailgun.ex:17:invalid_contract
The @spec for the function does not match the success typing of the function.

Function:
MyApp.Mailer.Mailgun.BambooMailer.deliver_later/1

Success typing:
@spec deliver_later(_) ::
  {:error,
   <<_::64, _::size(8)>>
   | %{
       :__exception__ => true,
       :__struct__ => Bamboo.EmptyFromAddressError | Bamboo.NilRecipientsError,
       :message => <<_::64, _::size(8)>>
     }}
  | {:ok,
     %{
       :bcc => {binary(), binary()},
       :cc => {binary(), binary()},
       :from => {binary(), binary()},
       :to => {binary(), binary()},
       _ => _
     }}

________________________________________________________________________________
done (warnings were emitted)

This PR fixes that spec error.

[lasseebert]

Note that this is not part of #583, since it seems that the generated functions inside __using__ are not dialyzed in that PR.

[germsvel]

Good catch! Thanks for opening this PR and thanks for the little note disambiguating from #583. Those kinds of notes are so helpful. 🎉