fixed typo in custom type casting docs (#229)

Author: Fivell

README.md

@@ -468,7 +468,7 @@ require 'money'
 class MyMoneyCaster
   def self.cast(value, default)
     begin
-      Money.new(1000, "USD")
+      Money.new(value, "USD")
     rescue ArgumentError
       default
     end

lib/json_api_client/schema.rb

@@ -60,7 +60,7 @@ class TypeFactory
       #   class MyMoneyCaster
       #      def self.cast(value, default)
       #         begin
-      #           Money.new(1000, "USD")
+      #           Money.new(value, "USD")
       #         rescue ArgumentError
       #           default
       #         end