Revert change in guide example code

In this example the User model is the default one from Laravel, not the default Twill user model or a custom Twill user model.

Author: ifox

docs/content/2_guides/manage_frontend_user_profiles_from_twill/Profile.php

@@ -15,6 +15,6 @@ class Profile extends Model
 
     public function user()
     {
-        return $this->belongsTo(twillModel('user'));
+        return $this->belongsTo(User::class);
     }
 }