fix path for BillingInfo.find

Right now this raises a `Recurly::Resource::NotFound` exception:

    Recurly::BillingInfo.find('code')

But this returns the correct BillingInfo resource:

    Recurly::BillingInfo.find(
      'https://api.recurly.com/v2/accounts/code/billing_info'
    )

This commit fixes the first example.