Docs: Fix incorrect theme slug in README (wprog-support → wporg-support)

[manhphuc]

This PR corrects a small typo in the README file.

The setup instructions currently reference the theme slug wprog-support, which does not exist.
The correct theme slug used on WordPress.org is wporg-support.

Updating this improves accuracy and prevents confusion for contributors setting up HelpHub locally.

No other files were modified.