react-express-mongodb: rename server to backend

The goal here is to keep the same structure than the other examples.
Also, considering that it has a "frontend" folder, the equivalent for
the server should be "backend".

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

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

@@ -0,0 +1,14 @@
+/**
+ * Created by Syed Afzal
+ */
+const mongoose = require('mongoose');
+
+var Todo = mongoose.model('Todo', {
+    text : {
+        type: String,
+        trim: true,
+        required: true
+    }
+});
+
+module.exports = {Todo};