Updated paths to UMD builds for React and React-DOM in 'How to Contri…

…bute' page (reactjs#3523)

content/docs/how-to-contribute.md

@@ -130,7 +130,7 @@ First, run `yarn build`. This will produce pre-built bundles in `build` folder,
 
 The easiest way to try your changes is to run `yarn build react/index,react-dom/index --type=UMD` and then open `fixtures/packaging/babel-standalone/dev.html`. This file already uses `react.development.js` from the `build` folder so it will pick up your changes.
 
-If you want to try your changes in your existing React project, you may copy `build/dist/react.development.js`, `build/dist/react-dom.development.js`, or any other build products into your app and use them instead of the stable version. 
+If you want to try your changes in your existing React project, you may copy `build/node_modules/react/umd/react.development.js`, `build/node_modules/react-dom/umd/react-dom.development.js`, or any other build products into your app and use them instead of the stable version. 
 
 If your project uses React from npm, you may delete `react` and `react-dom` in its dependencies and use `yarn link` to point them to your local `build` folder. Note that **instead of `--type=UMD` you'll want to pass `--type=NODE` when building**. You'll also need to build the `scheduler` package: