Update Postmark API token variable name in mail configuration (#10914)

* Update Postmark API token variable name in mail configuration

This doesn't match actual `services.php`

there it's referenced as 'POSTMARK_TOKEN' not 'POSTMARK_API_TOKEN'

```
 'postmark' => [
        'token' => env('POSTMARK_TOKEN'),
    ],
```

* Update Postmark API key environment variable

---------

Co-authored-by: Taylor Otwell <taylor@laravel.com>

Author: bomshteyn

mail.md

@@ -111,7 +111,7 @@ Next, set the `default` option in your application's `config/mail.php` configura
 
 ```php
 'postmark' => [
-    'key' => env('POSTMARK_API_TOKEN'),
+    'key' => env('POSTMARK_API_KEY'),
 ],
 ```