Display running command when showing help on stellar contract invoke.

[fnando]

What

Display running command when showing help on stellar contract invoke.

Before:

$ stellar contract invoke --id hello
Usage: CDTQG6AHKP452WCOPK6EKU6W7K236TSDMVZSQXJDHZYGIAGQUEZ446YW [COMMAND]

Commands:
  hello
  help   Print this message or the help of the given subcommand(s)

Options:
  -h, --help
          Print help

After:

$ stellar contract invoke --id hello
Usage: stellar contract invoke --id hello -- [COMMAND]

Commands:
  hello
  help   Print this message or the help of the given subcommand(s)

Options:
  -h, --help
          Print help

$ stellar contract invoke --id hello -- --help
Usage: stellar contract invoke --id hello -- [COMMAND]

Commands:
  hello
  help   Print this message or the help of the given subcommand(s)

Options:
  -h, --help  Print help

Why

Close #1858

Known limitations

N/A