Fix Chrome OS spelling in documentation

chrome/common/extensions/docs/templates/articles/app_codelab_alarms.html

 <h1 id="add-alarms-notifications">
   <span class="h1-step">Step 3:</span>
   Add Alarms and Notifications
 </h1>
 
 <p class="note">
   <strong>Want to start fresh from here?</strong>
   Find the previous step's code in the <a href="https://github.com/mangini/io13-codelab/archive/master.zip">reference code zip</a> under <strong><em>cheat_code > solution_for_step2</strong></em>.
 </p>
 
@@ skipping 138 matching lines @@
 in the Console every time the alarm "rings":</p>
 
 <figure>
   <img src="{{static}}/images/app_codelab/alarm-console-logs.png" alt="Alarm messages printing to the Console"/>
 </figure>
 
 <p>You should notice that:</p>
 
 <ul>
   <li>Even when you close the Todo app window, the alarms will keep coming.</li>
-  <li>On platforms other than ChromeOS, if you completely close all Chrome browser instances, alarms won't trigger.</li>
+  <li>On platforms other than Chrome OS, if you completely close all Chrome browser instances, alarms won't trigger.</li>
 </ul>
 
 <p>Let's go over some of the pieces in <em>alarms.js</em> that use <code>chrome.alarms</code> methods one by one.</p>
 
 <h3 id="create-alarms">Create alarms</h3>
 
 <p>In <code>createAlarm()</code>, use the <a href="/apps/alarms#method-create"><code>chrome.alarms.create()</code></a> API to create an alarm when <strong>Activate alarm</strong> is toggled.</p>
 
 <pre data-filename="alarms.js">
 chrome.alarms.create(alarmName, {delayInMinutes: 0.1, periodInMinutes: 0.1});
@@ skipping 239 matching lines @@
     <a href="/apps/notifications" title="Read 'chrome.notifications.clear()' in the Chrome developer docs">chrome.notifications.clear()</a>
     <a href="#clear-notification" class="anchor-link-icon" title="This feature mentioned in 'Clear the notification'">&#8593;</a>
   </li> -->
   <!-- <li>
     <a href="/apps/desktop_notifications" title="Read 'Rich Notifications' in the Chrome developer docs">Rich Notifications</a>
     <a href="#rich-notifications" class="anchor-link-icon" title="This feature mentioned in 'Learn more about rich notifications'">&#8593;</a>
   </li> -->
 </ul>
 
 <p>Ready to continue onto the next step? Go to <a href="app_codelab_webview.html">Step 4 - Open external links with a webview &raquo;</a></p>