Update the URL of Yarn's installation instructions (#4723)

Author: xfq

README.md

@@ -8,7 +8,7 @@ This repo contains the source code and documentation powering [reactjs.org](http
 
 1. Git
 1. Node: any 12.x version starting with v12.0.0 or greater
-1. Yarn: See [Yarn website for installation instructions](https://yarnpkg.com/lang/en/docs/install/)
+1. Yarn v1: See [Yarn website for installation instructions](https://yarnpkg.com/lang/en/docs/install/)
 1. A fork of the repo (for any contributions)
 1. A clone of the [reactjs.org repo](https://github.com/reactjs/reactjs.org) on your local machine