Update ProseMirror install instructions

prose/README.md

@@ -1,7 +1,7 @@
 # Building
 
 * Run `npm install` to download dependencies.
-* Run `npm run build` to build a production script in `dist/` or run
+* Run `npm run-script build` to build a production script in `../static/js/` or run
 `npm run develop` to build and watch for changes. You can use `prose.html`
 to test your development changes.
 * Manually copy the file `prose.bundle.js` to `static/js/`. _To be automated_