git submodule--helper clone usage stated that paths were added after the
[--] argument. The actual implementation required use of --path argument
and only supports one path at a time. Update the usage string to match
the current implementation.
Signed-off-by: Jacob Keller <jacob.keller@gmail.com> Reviewed-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>