Remove customer from params typespec in CustomerBalanceTransaction.create

[ascandella]

Prior to this PR, the only valid way to use this function would return a dialyzer error.

It's invalid to pass the customer ID as both part of the URL (which
this function does) and in the body. Doing so leads to a Stripe
error:

iex(1)> Stripe.CustomerBalanceTransaction.create("cus_JO4D6sa5xl4Uk7", %{amount: -100, currency: "USD", customer: "cus_JO4D6sa5xl4Uk7"})
{:error,
 %Stripe.Error{
   code: :invalid_request_error,
   extra: %{
     http_status: 400,
     param: :customer,
     raw_error: %{
       "message" => "A parameter provided in the URL (customer) was repeated as a GET or POST parameter. You can only provide this information as a portion of the URL.",
       "param" => "customer",
       "type" => "invalid_request_error"
     }
   },
   message: "A parameter provided in the URL (customer) was repeated as a GET or POST parameter. You can only provide this information as a portion of the URL.",
   request_id: nil,
   source: :stripe,
   user_message: nil
 }}

Whereas passing it as the first (required) positional argument works
fine:

iex(2)> Stripe.CustomerBalanceTransaction.create("cus_JO4D6sa5xl4Uk7", %{amount: -100, currency: "USD"})
{:ok,
 %Stripe.CustomerBalanceTransaction{
   amount: -100,
   created: 1619635421,
   credit_note: nil,
   currency: "usd",
   customer: "cus_JO4D6sa5xl4Uk7",
   description: nil,
   ending_balance: -100,
   id: "cbtxn_1IlIMLFvgMYR2X5ykmdTnR5H",
   invoice: nil,
   livemode: false,
   metadata: %{},
   object: "customer_balance_transaction",
   type: "adjustment"
 }}

(the customer here is a throwaway user in my test account)

[coveralls]

Coverage remained the same at 90.712% when pulling 8e42df6 on ascandella:fix-customer-balance-typespec into ce0f23f on code-corps:master.

[snewcomer]

Thanks!