Replace link to the line on Github with Rdoc link

The previous link tries to point to the exact line where a method is defined.
This link is weak and will point to a different place if someone adds or removes
code in that file.

As a solution, the link will point to the method documentation on rdoc.info

Author: ceritium

guides/source/rails_application_templates.md

@@ -271,9 +271,10 @@ Advanced Usage
 --------------
 
 The application template is evaluated in the context of a
-`Rails::Generators::AppGenerator` instance. It uses the `apply` action
-provided by
-[Thor](https://github.com/erikhuda/thor/blob/master/lib/thor/actions.rb#L207).
+`Rails::Generators::AppGenerator` instance. It uses the
+[`apply`](https://rdoc.info/github/wycats/thor/Thor/Actions#apply-instance_method)
+action provided by Thor.
+
 This means you can extend and change the instance to match your needs.
 
 For example by overwriting the `source_paths` method to contain the