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>

Documentation/doc-guide/parse-headers.rst

@@ -32,7 +32,7 @@ SYNOPSIS
 
 \ **parse_headers.pl**\  [<options>] <C_FILE> <OUT_FILE> [<EXCEPTIONS_FILE>]
 
-Where <options> can be: --debug, --help or --man.
+Where <options> can be: --debug, --help or --usage.
 
 
 OPTIONS

Documentation/sphinx/parse-headers.pl

@@ -344,7 +344,7 @@ enums and defines and create cross-references to a Sphinx book.
 
 B<parse_headers.pl> [<options>] <C_FILE> <OUT_FILE> [<EXCEPTIONS_FILE>]
 
-Where <options> can be: --debug, --help or --man.
+Where <options> can be: --debug, --help or --usage.
 
 =head1 OPTIONS