react-express-mongodb: lint sources

to fix some syntax issues in the existing code, the commit
put in place prettier and format the code

Signed-off-by: Jérémie Drouet <jeremie.drouet@gmail.com>

react-express-mongodb/backend/models/todos/todo.js

@@ -3,7 +3,7 @@
  */
 const mongoose = require('mongoose');
 
-var Todo = mongoose.model('Todo', {
+const Todo = mongoose.model('Todo', {
     text : {
         type: String,
         trim: true,