Update create card spec (#382)

* updated create card spec

I was getting confused looking at the docs for create card because it does not mention how to pass customer so I think adding that to the params spec makes sense as it is that way elsewhere in the code.

* Update card.ex

* reordered

reordered params so it looks better.

lib/stripe/payment_methods/card.ex

@@ -100,8 +100,9 @@ defmodule Stripe.Card do
   """
   @spec create(params, Keyword.t()) :: {:ok, t} | {:error, Stripe.Error.t()}
         when params: %{
+              :customer => Stripe.id() | Stripe.Customer.t(),
+              :source => Stripe.id() | Stripe.Source.t(),
               optional(:metadata) => Stripe.Types.metadata(),
-              optional(:source) => Stripe.id() | Stripe.Source.t(),
             }
   def create(%{customer: _, source: _} = params, opts \\ []) do
     new_request(opts)