API key command placeholder URL is wrong if using custom endpoint

If I have changed the API endpoint to not use the official instance, this placeholder text still mentions testaustime.fi
https://github.com/Testaustime/testaustime-vscode/blob/3a7f4c8ad1c189cc92ad835dd53a46ffa1bea59d/src/testaustime.ts#L126

<img width="921" height="257" alt="Image" src="https://github.com/user-attachments/assets/5847fe88-425b-426c-b8bb-097e0461f8c6" />