parse-options.c: fix documentation syntax of optional arguments

When an argument for an option is optional, short options don't need a
space between the option and the argument, and long options need a "=".
Otherwise, arguments are misinterpreted.

Signed-off-by: Michele Ballabio <barra_cuda@katamail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>