This PR addresses several broken Rubydoc links by updating them to their correct destinations.

I found the following three links were broken:
- ./guides/source/action_controller_overview.md:1203
- ./guides/source/action_controller_overview.md:1274
- ./guides/source/testing.md:1864

Author: Judeeeee

guides/source/action_controller_overview.md

@@ -1200,7 +1200,7 @@ the client. This section describes the purpose of some of the properties of the
 
 To get a full list of the available methods, refer to the [Rails API
 documentation](https://api.rubyonrails.org/classes/ActionDispatch/Request.html)
-and [Rack](https://www.rubydoc.info/github/rack/rack/Rack/Request)
+and [Rack](https://www.rubydoc.info/gems/rack/Rack/Request)
 documentation.
 
 | Property of `request`                     | Purpose                                                                          |
@@ -1271,4 +1271,4 @@ Here are some of the properties of the `response` object:
 To get a full list of the available methods, refer to the [Rails API
 documentation](https://api.rubyonrails.org/classes/ActionDispatch/Response.html)
 and [Rack
-Documentation](https://www.rubydoc.info/github/rack/rack/Rack/Response).
+Documentation](https://www.rubydoc.info/gems/rack/Rack/Response).

guides/source/testing.md

@@ -1861,7 +1861,7 @@ documentation](https://github.com/rails/rails-dom-testing).
 In order to integrate with [rails-dom-testing][], tests that inherit from
 `ActionView::TestCase` declare a `document_root_element` method that returns the
 rendered content as an instance of a
-[Nokogiri::XML::Node](https://www.rubydoc.info/github/sparklemotion/nokogiri/Nokogiri/XML/Node):
+[Nokogiri::XML::Node](https://www.rubydoc.info/gems/nokogiri/Nokogiri/XML/Node):
 
 ```ruby
 test "renders a link to itself" do