Docs: Create persistent file example provides incorrect code

# Bug Report

## Problem

https://github.com/apache/cordova-plugin-file#create-a-persistent-file-

`LocalFileSystem.PERSISTENT` does not exists. It should be `window.PERSISTENT`, as confirmed by a support ticket https://github.com/apache/cordova-plugin-file/issues/369

The documentation should be updated to `window.PERSISTENT`. Note that, the `window` namespace is also used in [create temporary file example](https://github.com/apache/cordova-plugin-file#create-a-temporary-file)

### Command or Code

`LocalFileSystem.PERSISTENT`

### Environment, Platform, Device

All platforms

### Version information
cordova-plugin-file 6.0.3-dev - not sure when the documentation was written however.

## Checklist

- [x] I searched for existing GitHub issues
- [x] I updated all Cordova tooling to most recent version
- [x] I included all the necessary information above

Open the linked “Create a persistent file” documentation example and compare it with the linked temporary-file example. Verify the documented namespace against the issue’s support-ticket reference, update the incorrect example, and check that the persistent example is consistent with the temporary one.

Written by the indexing model from the issue text.