docs: Update codes in docs to use require('electron')

docs/api/screen.md

@@ -6,20 +6,20 @@ position, etc. You should not use this module until the `ready` event of the
 
 `screen` is an [EventEmitter](http://nodejs.org/api/events.html#events_class_events_eventemitter).
 
-**Note:** In the renderer / DevTools, `window.screen` is a reserved
-DOM property, so writing `var screen = require('screen')` will not work. In our
-examples below, we use `electronScreen` as the variable name instead.
-
+**Note:** In the renderer / DevTools, `window.screen` is a reserved DOM
+property, so writing `var screen = require('electron').screen` will not work.
+In our examples below, we use `electronScreen` as the variable name instead.
 An example of creating a window that fills the whole screen:
 
 ```javascript
-var app = require('app');
-var BrowserWindow = require('browser-window');
+const electron = require('electron');
+const app = electron.app;
+const BrowserWindow = electron.BrowserWindow;
 
 var mainWindow;
 
 app.on('ready', function() {
-  var electronScreen = require('screen');
+  var electronScreen = electron.screen;
   var size = electronScreen.getPrimaryDisplay().workAreaSize;
   mainWindow = new BrowserWindow({ width: size.width, height: size.height });
 });
@@ -28,13 +28,14 @@ app.on('ready', function() {
 Another example of creating a window in the external display:
 
 ```javascript
-var app = require('app');
-var BrowserWindow = require('browser-window');
+const electron = require('electron');
+const app = electron.app;
+const BrowserWindow = electron.BrowserWindow;
 
 var mainWindow;
 
 app.on('ready', function() {
-  var electronScreen = require('screen');
+  var electronScreen = electron.screen;
   var displays = electronScreen.getAllDisplays();
   var externalDisplay = null;
   for (var i in displays) {