Better examples for fresh_when and stale?

It is clearer and closer to reality to use `@article.updated_at` as
the `:last_modified` parameter of `fresh_when` and `stale?`.

Using `@article.created_at` would result in the cache never expiring,
since the creation timestamp never changes.

[ci skip]

actionpack/lib/action_controller/metal/conditional_get.rb

@@ -51,7 +51,7 @@ def etag(&etagger)
     #
     #   def show
     #     @article = Article.find(params[:id])
-    #     fresh_when(etag: @article, last_modified: @article.created_at, public: true)
+    #     fresh_when(etag: @article, last_modified: @article.updated_at, public: true)
     #   end
     #
     # This will render the show template if the request isn't sending a matching ETag or
@@ -115,7 +115,7 @@ def fresh_when(record_or_options, additional_options = {})
     #   def show
     #     @article = Article.find(params[:id])
     #
-    #     if stale?(etag: @article, last_modified: @article.created_at)
+    #     if stale?(etag: @article, last_modified: @article.updated_at)
     #       @statistics = @article.really_expensive_call
     #       respond_to do |format|
     #         # all the supported formats