fix(app.js): Change config.maintenance to realtime.maintenance

app.js

@@ -229,7 +229,7 @@ process.on('uncaughtException', function (err) {
 // install exit handler
 function handleTermSignals () {
   logger.info('hackmd has been killed by signal, try to exit gracefully...')
-  config.maintenance = true
+  realtime.maintenance = true
   // disconnect all socket.io clients
   Object.keys(io.sockets.sockets).forEach(function (key) {
     var socket = io.sockets.sockets[key]