docs: Fix link to system secret rotation

[sawadashota]

Proposed changes

The following link no longer exists
https://www.ory.sh/docs/hydra/advanced#system-secret-rotation

New link is here
https://www.ory.sh/docs/hydra/advanced#rotation-of-hmac-token-signing-and-database-and-cookie-encryption-keys

Checklist

• I have read the contributing guidelines
• I have read the security policy
• I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security
  vulnerability, I confirm that I got green light (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)
• I have documented my changes in the developer guide (if appropriate)

[aeneasr]

Thank you!