docs: add align: 'center' to default boxenOpts (#86)

readme.md

@@ -128,7 +128,7 @@ The message that will be shown when an update is available.
 ### options.boxenOpts
 
 Type: `object`<br>
-Default: `{ padding: 1, margin: 1, borderColor: 'yellow', borderStyle: 'round' }` ([See the screen shot above](https://github.com/yeoman/update-notifier#update-notifier-))
+Default: `{ padding: 1, margin: 1, align: 'center', borderColor: 'yellow', borderStyle: 'round' }` ([See the screen shot above](https://github.com/yeoman/update-notifier#update-notifier-))
 
 The object that will be passed to [boxen](https://github.com/sindresorhus/boxen).