Fix links to beginner friendly issues

Summary:
<!--
Thank you for sending the PR! We appreciate you spending the time to work on these changes.

Help us understand your motivation by explaining why you decided to make this change.

You can learn more about contributing to React Native here: http://facebook.github.io/react-native/docs/contributing.html

Happy contributing!

-->

While I was reading README I found wrong links to `beginner friendly issues` so just fixed them.
Closes https://github.com/facebook/react-native/pull/17579

Differential Revision: D6719845

Pulled By: shergin

fbshipit-source-id: 5c3290f70efbd8c40c2d3482c23c198ab45d510e