Update VONAGE_SMS_FROM documentation (#8555)

* Update VONAGE_SMS_FROM documentation

Make it clearer that you should set this environment variable for Vonage notifications to work

* Update notifications.md

---------

Co-authored-by: Taylor Otwell <[email protected]>

Author: nicolus

notifications.md

@@ -1016,7 +1016,7 @@ Sending SMS notifications in Laravel is powered by [Vonage](https://www.vonage.c
 
 The package includes a [configuration file](https://github.com/laravel/vonage-notification-channel/blob/3.x/config/vonage.php). However, you are not required to export this configuration file to your own application. You can simply use the `VONAGE_KEY` and `VONAGE_SECRET` environment variables to define your Vonage public and secret keys.
 
-After defining your keys, you may set a `VONAGE_SMS_FROM` environment variable that defines the phone number that your SMS messages should be sent from by default. You may generate this phone number within the Vonage control panel:
+After defining your keys, you should set a `VONAGE_SMS_FROM` environment variable that defines the phone number that your SMS messages should be sent from by default. You may generate this phone number within the Vonage control panel:
 
     VONAGE_SMS_FROM=15556666666