doc:sphinx: fix parse-header description

The description speaks about the option ``--man`` but it
does not exist. Instead, there is the option ``--usage``

    $ ./Documentation/sphinx/parse-headers.pl --man
    Unknown option: man
    Usage:
        parse_headers.pl [<options>] <C_FILE> <OUT_FILE> [<EXCEPTIONS_FILE>]

        Where <options> can be: --debug, --help or --man.

Signed-off-by: Federico Vaga <federico.vaga@vaga.pv.it>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>