fix: typespec in generated code

[ymtszw]

Hi. This is a small typespec fix.

@spec for deliver_now/2 when using Bamboo.Mailer defines tuple
for when we call it with response: true option.
However the slots in the tuple are misaligned from actual implementation,
causing dialyzer warning in our code.

Impl is:

      response = adapter.deliver(email, config)
      {email, response}

https://github.com/thoughtbot/bamboo/blob/8c5780319c5e2c936d04a2e1d9594be20a525c9b/lib/bamboo/mailer.ex#L149-L150

[germsvel]

Thanks for catching this! 🔍 👀