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

docs/api/power-monitor.md

@@ -1,4 +1,4 @@
-# power-monitor
+# powerMonitor
 
 The `power-monitor` module is used to monitor power state changes. You can
 only use it in the main process. You should not use this module until the `ready`
@@ -7,10 +7,8 @@ event of the `app` module is emitted.
 For example:
 
 ```javascript
-var app = require('app');
-
 app.on('ready', function() {
-  require('power-monitor').on('suspend', function() {
+  require('electron').powerMonitor.on('suspend', function() {
     console.log('The system is going to sleep');
   });
 });