Node.js sample: Use index.js instead of app.js. (#849)

* Use index.js instead of app.js.

index.js is the more idiomatic for Node.js developers.
Indeed, it is the default value that npm start uses if no start script is provided.

* Update package.json

* Update README.md

* Update package.json

serving/samples/helloworld-nodejs/README.md

@@ -19,9 +19,7 @@ While you can clone all of the code from this directory, hello world apps are
 generally more useful if you build them step-by-step. The following instructions
 recreate the source files from this folder.
 
-1. Create a new directory and initalize `npm`. You can accept the defaults, but
-   change the entry point to `app.js` to be consistent with the sample code
-   here.
+1. Create a new directory and initalize `npm`. 
 
    ```shell
    npm init
@@ -29,7 +27,7 @@ recreate the source files from this folder.
    package name: (helloworld-nodejs)
    version: (1.0.0)
    description:
-   entry point: (index.js) app.js
+   entry point: (index.js)
    test command:
    git repository:
    keywords:
@@ -43,7 +41,7 @@ recreate the source files from this folder.
    npm install express --save
    ```
 
-1. Create a new file named `app.js` and paste the following code:
+1. Create a new file named `index.js` and paste the following code:
 
    ```js
    const express = require("express");
@@ -69,9 +67,9 @@ recreate the source files from this folder.
      "name": "knative-serving-helloworld",
      "version": "1.0.0",
      "description": "",
-     "main": "app.js",
+     "main": "index.js",
      "scripts": {
-       "start": "node app.js"
+       "start": "node index.js"
      },
      "author": "",
      "license": "Apache-2.0",

serving/samples/helloworld-nodejs/package.json

@@ -2,9 +2,9 @@
   "name": "knative-serving-helloworld",
   "version": "1.0.0",
   "description": "Simple hello world sample in Node",
-  "main": "app.js",
+  "main": "index.js",
   "scripts": {
-    "start": "node app.js"
+    "start": "node index.js"
   },
   "author": "",
   "license": "Apache-2.0",