Remove incorrect Rack documentation example

`Sprockets` does not respond to the call method, so I changed the
example to use a fake `MyRackApp`.

Closes #22579.

[ci skip]

guides/source/routing.md

@@ -812,10 +812,10 @@ In all of these cases, if you don't provide the leading host (`http://www.exampl
 Instead of a String like `'articles#index'`, which corresponds to the `index` action in the `ArticlesController`, you can specify any [Rack application](rails_on_rack.html) as the endpoint for a matcher:
 
 ```ruby
-match '/application.js', to: Sprockets, via: :all
+match '/application.js', to: MyRackApp, via: :all
 ```
 
-As long as `Sprockets` responds to `call` and returns a `[status, headers, body]`, the router won't know the difference between the Rack application and an action. This is an appropriate use of `via: :all`, as you will want to allow your Rack application to handle all verbs as it considers appropriate.
+As long as `MyRackApp` responds to `call` and returns a `[status, headers, body]`, the router won't know the difference between the Rack application and an action. This is an appropriate use of `via: :all`, as you will want to allow your Rack application to handle all verbs as it considers appropriate.
 
 NOTE: For the curious, `'articles#index'` actually expands out to `ArticlesController.action(:index)`, which returns a valid Rack application.