Update index.html

Author: rbrtsmith

demo/index.html

@@ -16,7 +16,7 @@ <h1>Nebula CSS &mdash; Demo</h1>
       <p>You are encouraged to use your browser dev tools to see what markup structure these components require</p>
       <p><strong>Note:</strong> Anything prefixed with <code>demo-</code> is for demonstration purposes only and does not ship with the framework</p>
       <h2>Install via NPM</h2>
-      <pre><code>npm install --save nebula-css</code></pre>
+      <pre><code>yarn add nebula-css</code></pre>
     </div>
   </div>