Update notification icon (#4343)

templates/base/head.tmpl

@@ -174,7 +174,7 @@
 						<div class="right stackable menu">
 							<a href="{{AppSubUrl}}/notifications" class="item poping up" data-content='{{.i18n.Tr "notifications"}}' data-variation="tiny inverted">
 								<span class="text">
-									<i class="fitted octicon octicon-inbox"></i>
+									<i class="fitted octicon octicon-bell"></i>
 									<span class="sr-mobile-only">{{.i18n.Tr "notifications"}}</span>
 
 									{{if .NotificationUnreadCount}}