Fix Yarn usage instructions

[fson]

I fixed the yarnpkg alias usage in #329, but as a regression, the command is now written as yarnpkg in the usage instructions 😳

Success! Created my-crna-app at /Users/ville/Projects/my-crna-app
Inside that directory, you can run several commands:

  yarnpkg start
    Starts the development server so you can open your app in the Expo
    app on your phone.

  yarnpkg run ios
...

This PR changes the instructions back to yarn while keeping yarnpkg as the command that's actually invoked.

Test plan

Ran npm run build && npm pack in the react-scripts folder and created an app using CRNA:

create-react-native-app my-crna-app5 --scripts-version file:/path-to-react-native-scripts.tgz

Running ps in another terminal during installation displayed yarnpkg as the running process and instructions were displayed correctly in the end: