Include optional public-url flag in build instruction within getting started docs

docs/reference/src/platforms/web.md

@@ -77,7 +77,7 @@ trunk serve
 To build our app and publish it to Github:
 
 - Make sure Github Pages is set up for your repo
-- Build your app with `trunk build --release`
+- Build your app with `trunk build --release` (include `--public-url <repo-name>` to update asset prefixes if using a project site)
 - Move your generated HTML/CSS/JS/Wasm from `dist` into the folder configured for Github Pages
 - Add and commit with git
 - Push to Github