README: Fix link to Admin Quick Start Guide

In the README, Item 2 of the Getting Started Section links to 
https://github.com/discourse/discourse/wiki/The-Discourse-Admin-Quick-Start-Guide which has a link pointing users to visit https://github.com/discourse/discourse/blob/master/docs/ADMIN-QUICK-START-GUIDE.md

This changes the README to directly link to https://github.com/discourse/discourse/blob/master/docs/ADMIN-QUICK-START-GUIDE.md