Fix Chrome OS spelling in documentation

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

 <h1 id="richNotifications">Rich Notifications</h1>
 
 <p>The <a href="https://developer.chrome.com/apps/notifications">rich notifications API</a>
 lets you create notifications using templates and
 show these notifications to users in the user's system tray:
 </p>
 
 <p><img src="{{static}}/images/notifications.png"
      alt="Notifications in system user tray">
 </p>
@@ skipping 41 matching lines @@
      alt="Progress notification">
 </p>
 
 <h2 id="behave">How they behave</h2>
 
 <p>
 Notifications show up in a user's system tray,
 and stay in the system tray until the user dismisses them.
 On most platforms,
 the bell icon is lit when there's new notifications;
-On ChromeOS,
+On Chrome OS,
 the system tray keeps a count of all new notifications.
 Once a users sees the notifications in the system tray,
 the count is reset to zero;
 </p>
 
 <p>
 Notifications can be assigned a priority between -2 to 2.
 Priorities < 0 are only shown in the center;
 priorities > 0 are shown for increasing duration and
 more high priority notifications can be displayed in the system tray.
@@ skipping 148 matching lines @@
 function replyBtnClick {
         //Write function to respond to user action.
 }
 </pre>
 
 <p>Consider including event listeners and handlers within the
   <a href="app_lifecycle#create_event_page">event page</a>,
 so that notifications can pop-up even when the app or extension isn't running.
 </p>