Fix deprecation message for Spree::CreditCard

[spaghetticode]

Description

Deprecation message on CreditCard#default is a bit misleading, as it
prints out CreditCard.default but it's referencing an instance method
not a class method.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message

[jacobherrington]

Good catch! I wonder if we do this elsewhere. 🤔

[spaghetticode]

@jacobherrington I think this is probably an isolated case, caused by cut & paste from the deprecation of the class method just a few lines above:

    def self.default
      Spree::Deprecation.warn("CreditCard.default is deprecated. Please use Spree::Wallet instead.")
      joins(:wallet_payment_sources).where(spree_wallet_payment_sources: { default: true })
    end