Fix command to install package

Fix the command to:
"composer require laravel-notification-channels/webpush" instead of "composer require laravel-notification-channels/web-push-notifications".

Author: nmfzone

README.md

@@ -17,7 +17,7 @@ This package makes it easy to send web push notifications with Laravel 5.3.
 You can install the package via composer:
 
 ``` bash
-composer require laravel-notification-channels/web-push-notifications
+composer require laravel-notification-channels/webpush
 ```
 
 First you must install the service provider: