Update UPGRADING.md (#152)

Fix documented path (asset/config/manifest.js -> app/assets/config/manifest.js) for clarity.

Author: paul-millar

UPGRADING.md

@@ -207,7 +207,7 @@ Start by following these steps:
 1. Remove `sprockets`, `sprockets-rails`, and `sass-rails` from the Gemfile and add `propshaft`;
 2. Run `./bin/bundle install`;
 3. Open `config/application.rb` and remove `config.assets.paths << Rails.root.join('app','assets')`;
-4. Remove `asset/config/manifest.js`.
+4. Remove `app/assets/config/manifest.js`.
 5. Replace all asset_helpers (`image_url`, `font_url`) in css files with standard `urls`.
 6. If you are importing only the frameworks you need (instead of `rails/all`), remove `require "sprockets/railtie"`;