[Urgent] `relay_invoke_method` method not working

## Summary
A typo occurred in the file changes for adding an external library integrity verification feature to the worker script of Caterpillar Proxy, written in PHP. The variable that should have been declared as `$resource_url` was mistakenly declared as `$required_url`.

The issue was urgently fixed and deployed.

Affected file:  
* `app/assets/php/index.php`