Docs: Add missing information about `format` parameter in sign command.

[icedevml]

Description

Checklist

Changes to the drivers

• [ ] (PR Author) The affected drivers were manually tested

Changes to CLI

• [ ] (PR Author) The change was manually tested with the CLI
• [ ] (PR Author) The affected CLI features are working with the standalone binary (at least one platform)
• [ ] (Checked by maintainer) The CLI test procedure was run by the project's maintainer

Changes to web library

• [ ] (PR Author) The change was manually tested with the web library included within a classic HTML application (flat libhalo.js)
• [ ] (PR Author) The change was manually tested with the web library included within an app based on frontend framework (React.js or similar based on webpack)
• [ ] (Checked by maintainer) The web test suite was run by the project's maintainer

Changes to nfc-manager driver

• [ ] (PR Author) The change was manually tested in React Native app
• [ ] (Checked by maintainer) The test suite was run through the test React Native project