docs: readme typos

Author: atymic

README.md

@@ -6,7 +6,7 @@
 [![Quality Score](https://img.shields.io/scrutinizer/g/laravel-notification-channels/fcm.svg?style=flat-square)](https://scrutinizer-ci.com/g/laravel-notification-channels/fcm)
 [![Total Downloads](https://img.shields.io/packagist/dt/laravel-notification-channels/fcm.svg?style=flat-square)](https://packagist.org/packages/laravel-notification-channels/fcm)
 
-This package makes it easy to send notifications using [Firebase Cloud Messaging](https://firebase.google.com/docs/cloud-messaging/) (FCM) with Laravel 5.3, 5.4, and 5.5.
+This package makes it easy to send notifications using [Firebase Cloud Messaging](https://firebase.google.com/docs/cloud-messaging/) (FCM) with Laravel 5.5+ & 6.x
 
 ## Contents