Fix deprecation message for Spree::CreditCard

Deprecation messages on `CreditCard#default` and `CreditCard#default=`
are a bit misleading, as they print out `CreditCard.default`
but they are actually referencing an instance method, not a class one.

core/app/models/spree/credit_card.rb

@@ -47,13 +47,13 @@ def self.default
     }.freeze
 
     def default
-      Spree::Deprecation.warn("CreditCard.default is deprecated. Please use user.wallet.default_wallet_payment_source instead.", caller)
+      Spree::Deprecation.warn("CreditCard#default is deprecated. Please use user.wallet.default_wallet_payment_source instead.", caller)
       return false if user.nil?
       user.wallet.default_wallet_payment_source.try!(:payment_source) == self
     end
 
     def default=(set_as_default)
-      Spree::Deprecation.warn("CreditCard.default= is deprecated. Please use user.wallet.default_wallet_payment_source= instead.", caller)
+      Spree::Deprecation.warn("CreditCard#default= is deprecated. Please use user.wallet.default_wallet_payment_source= instead.", caller)
       if user.nil?
         raise "Cannot set 'default' on a credit card without a user"
       elsif set_as_default # setting this card as default