MAGETWO-89776: Release notification minor issues

- update content_url

Author: Joan He

app/code/Magento/ReleaseNotification/etc/config.xml

@@ -9,7 +9,7 @@
     <default>
         <system>
             <release_notification>
-                <content_url>raw.githubusercontent.com/magento-arcticfoxes/release-notification/master</content_url>
+                <content_url>magento.com/release_notifications</content_url>
                 <use_https>1</use_https>
             </release_notification>
         </system>