fix: correct vendor:publish tag in README

[derrickobedgiu1]

Related #29

The installation instructions in the README reference the wrong tag name for publishing the config file.

# documented (broken)
php artisan vendor:publish --tag="laravel-powerbi-config"

# correct
php artisan vendor:publish --tag="powerbi-config"

The tag registered in PowerBIServiceProvider is powerbi-config. The wrong tag silently does nothing, leaving new users without the config file and no obvious error message.