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/config/config.json

@@ -0,0 +1,10 @@
+{
+  "test":{
+    "PORT": 3000,
+    "MONGODB_URI": "mongodb://mongo:27017/TodoAppTest"
+  },
+  "development":{
+    "PORT": 3000,
+    "MONGODB_URI": "mongodb://mongo:27017/TodoApp"
+  }
+}