Fix inconsistent custom format documentation

Prior to this commit, the documentation for setting a custom Format was
inconsistently mixing module-level and instance-level method
definitions.

There isn't existing precedent in the documentation for how to declare
custom formats that share behavior with first-party formats (like xml
and json). Since the [test suite][] utilizes a `class`-based approach
(rather than a `module`-based one), change the `Base` documentation to
follow that precedent.

[test suite]: https://github.com/rails/activeresource/blob/v6.2.0/test/fixtures/address.rb#L4-L15

Author: seanpdoyle

lib/active_resource/base.rb

@@ -105,10 +105,10 @@ module ActiveResource
   # To customize how attributes are encoded and decoded, declare a format and override
   # its +encode+ and +decode+ methods:
   #
-  #   module CamelcaseJsonFormat
-  #     extend ActiveResource::Formats[:json]
+  #   class CamelcaseJsonFormat
+  #     include ActiveResource::Formats[:json]
   #
-  #     def self.encode(resource, options = nil)
+  #     def encode(resource, options = nil)
   #       hash = resource.as_json(options)
   #       hash = hash.deep_transform_keys! { |key| key.camelcase(:lower) }
   #       super(hash)
@@ -120,7 +120,7 @@ module ActiveResource
   #     end
   #   end
   #
-  #   Person.format = CamelcaseJsonFormat
+  #   Person.format = CamelcaseJsonFormat.new
   #
   #   person = Person.new(first_name: "First", last_name: "Last")
   #   person.encode

test/fixtures/address.rb

@@ -5,7 +5,7 @@ class AddressXMLFormatter
   include ActiveResource::Formats::XmlFormat
 
   def decode(xml)
-    data = ActiveResource::Formats::XmlFormat.decode(xml)
+    data = super
     # process address fields
     data.each do |address|
       address["city_state"] = "#{address['city']}, #{address['state']}"