cmd: Append subcommands help into badge help message

[umatare5]

WHAT

This PR updates the Use field of the badge command to explicitly include the required subcommand names.
This makes the help output display the required argument for the first time, improving clarity for users.

Before:

❯ go run main.go badge --help
generate badge.

Usage:
  octocov badge [flags]

Flags:
      --config string   config file path
  -h, --help            help for badge
      --out string      output file path

After:

❯ go run main.go badge --help
generate badge.

Usage:
  octocov badge <coverage|ratio|time> [flags]

Flags:
      --config string   config file path
  -h, --help            help for badge
      --out string      output file path

WHY

• [Improvement] octocov badge help message don't show required subcommands #523

[k1LoW]

Thank you!