Revert "Update asset_pipeline.md"

This reverts commit afe67218.

Reason: This change was done by mistake. This option is at the `config`
object not in the `config.assets`.

See
https://github.com/rails/rails/blob/cfb371e96a2789790359ec1db779df490fd253d6/railties/lib/rails/application/configuration.rb#L16

guides/source/asset_pipeline.md

@@ -169,7 +169,7 @@ directory. Files in this directory are served by the Sprockets middleware.
 
 Assets can still be placed in the `public` hierarchy. Any assets under `public`
 will be served as static files by the application or web server when
-`config.assets.serve_static_files` is set to true. You should use `app/assets` for
+`config.serve_static_files` is set to true. You should use `app/assets` for
 files that must undergo some pre-processing before they are served.
 
 In production, Rails precompiles these files to `public/assets` by default. The