Update installation instructions for yarn

Change `yarn install` to `yarn add`

Author: ZackLyon

_posts/plotly_js/2015-07-21-plotlyjs-getting-started.html

@@ -12,7 +12,7 @@
         <h2>NPM</h2>
         <div class="content-box">
             <p>
-                You can <a href="https://www.npmjs.com/package/plotly.js" target="_blank">install Plotly.js from NPM</a> via <code>npm install plotly.js-dist</code> or <code>yarn install plotly.js-dist</code>
+                You can <a href="https://www.npmjs.com/package/plotly.js" target="_blank">install Plotly.js from NPM</a> via <code>npm install plotly.js-dist</code> or <code>yarn add plotly.js-dist</code>
             </p>
 
     </section>