replace npm with yarn in gett started

Author: iezer

source/about.html.erb

@@ -40,7 +40,7 @@ title: "A framework for creating ambitious web applications."
     <a href="https://guides.emberjs.com/<%= current_version %>/getting-started/quick-start/">View Quick Start guide</a>
   </div>
   <div class="bottom">
-    <p>$ npm install -g ember-cli</p>
+    <p>$ yarn global add ember-cli</p>
     <p>$ ember new ember-quickstart</p>
   </div>
 </div>