Update app.js (#247)

* Update app.js

As recommended by Mongoose for resolving deprecation warnings:
https://mongoosejs.com/docs/deprecations.html

* Update app.js

Just adding a comma to follow eslint rules

Co-authored-by: Kelly Burke <kellyburke101@gmail.com>

Author: NguyenDa18

server/app.js

@@ -28,6 +28,7 @@ const options = {
   useNewUrlParser: true,
   useCreateIndex: true,
   useFindAndModify: false,
+  useUnifiedTopology: true,
 };
 
 mongoose.connect(MONGO_URL, options);