msglist: Change the icon for the "Mark all messages as read" button a…

…t the bottom of the message list

lib/widgets/message_list.dart

@@ -855,7 +855,7 @@ class _MarkAsReadWidgetState extends State<MarkAsReadWidget> {
                 backgroundColor: WidgetStatePropertyAll(messageListTheme.unreadMarker),
               ),
               onPressed: _loading ? null : () => _handlePress(context),
-              icon: const Icon(Icons.playlist_add_check),
+              icon: const Icon(ZulipIcons.message_checked),
               label: Text(zulipLocalizations.markAllAsReadLabel))))));
   }
 }