fix(docs): correct hmackey config section from [metrics] to [Application Options]

<img width="1592" height="544" alt="Image" src="https://github.com/user-attachments/assets/f922f9ef-4b48-40c1-a4e2-d41c909871b7" />

https://github.com/babylonlabs-io/finality-provider/blob/bbf99e5944f8582195a735f490bd559b06dcaf53/docs/hmac-security.md?plain=1#L52

