Update how-to-contribute.md

Author: gaearon

content/docs/how-to-contribute.md

@@ -115,13 +115,13 @@ Then, you can run several commands:
 * `yarn debug-test` is just like `yarn test` but with a debugger. Open `chrome://inspect` and press "Inspect".
 * `yarn flow` runs the [Flow](https://flowtype.org/) typechecks.
 * `yarn build` creates a `build` folder with all the packages.
-* `yarn build core,dom --type=UMD` creates UMD builds of just React and ReactDOM.
+* `yarn build react/index,react-dom/index --type=UMD` creates UMD builds of just React and ReactDOM.
 
 We recommend running `yarn test` (or its variations above) to make sure you don't introduce any regressions as you work on your change. However it can be handy to try your build of React in a real project.
 
 First, run `yarn build`. This will produce pre-built bundles in `build` folder, as well as prepare npm packages inside `build/packages`.
 
-The easiest way to try your changes is to run `yarn build core,dom --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.
+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 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: