add to mongoose code example

Example 1: node js + mongoose

await mongoose.connect('mongodb://localhost/my_database', {  useNewUrlParser: true,  useUnifiedTopology: true});

Example 2: node js + mongoose

const db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function() {
  // we're connected!
});