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/frontend/package.json

@@ -17,7 +17,7 @@
     "fsevents": "^2.1.2"
   },
   "scripts": {
-    "start": "PORT=5000 react-scripts start",
+    "start": "react-scripts start",
     "build": "react-scripts build",
     "test": "react-scripts test",
     "eject": "react-scripts eject"
@@ -25,7 +25,7 @@
   "eslintConfig": {
     "extends": "react-app"
   },
-  "proxy": "http://server:3000",
+  "proxy": "http://backend:3000",
   "browserslist": {
     "production": [
       ">0.2%",