Change PUSHER enviroment variable names

pusher.com's copy and paste .env snippet uses enviroment variables that
all start with "PUSHER_APP". This commit brings the config files in line
with pusher.com

.env.example

@@ -28,5 +28,5 @@ MAIL_PASSWORD=null
 MAIL_ENCRYPTION=null
 
 PUSHER_APP_ID=
-PUSHER_KEY=
-PUSHER_SECRET=
+PUSHER_APP_KEY=
+PUSHER_APP_SECRET=

config/broadcasting.php

@@ -32,8 +32,8 @@ return [
 
         'pusher' => [
             'driver' => 'pusher',
-            'key' => env('PUSHER_KEY'),
-            'secret' => env('PUSHER_SECRET'),
+            'key' => env('PUSHER_APP_KEY'),
+            'secret' => env('PUSHER_APP_SECRET'),
             'app_id' => env('PUSHER_APP_ID'),
             'options' => [
                 //