FB_VIRTUAL_BOOL_MQTT: fixing faulty state topic in docs

[MichielVanwelsenaere]

Pull Request Checklist

Check if the tasks below are relevant for your PR, to mark an item as completed use [X].

• [ ] Updated the PLCOpen XML export file (only relevant on code change).
• [ ] Updated the CoDeSys V3 export file (only relevant on code change).
• [x] Updated or created any relevant documentation.

Related issues

• Fixes #115