Update README to Use Correct Config Publish Tag

[kaspernowak]

Summary

This PR updates the documentation to reflect the correct publish tag for the package configuration file.

Details

• The previous instructions used --tag="config" for publishing the config file, which may have worked in earlier versions or with different package setups.
• With the current setup (using Spatie Laravel Package Tools), the config file is now registered with the tag bexio-config instead of the generic config.
• The README has been updated to instruct users to use --tag="bexio-config" when publishing the config file.

Previous Experience

Previously, following the old instructions and running:

php artisan vendor:publish --provider="CodebarAg\Bexio\BexioServiceProvider" --tag="config"

would result in the message:

INFO No publishable resources for tag [config].

and the config file would not be published.

Motivation

There has been potential changes in how config files are registered and tagged for publishing, possibly due to updates in package tools or Laravel itself. The old instructions no longer work as expected, but the new tag works reliably. This PR ensures the documentation is accurate and helps users avoid confusion.

[RhysLees]

Thanks @kaspernowak !

[kaspernowak]

My pleasure @RhysLees