man: fix hyphen used as minus sign

Report from Debian's lintian:

 This manual page seems to contain a hyphen where a minus sign was
 intended. By default, "-" chars are interpreted as hyphens (U+2010) by
 groff, not as minus signs (U+002D). Since options to programs use minus
 signs (U+002D), this means for example in UTF-8 locales that you cannot
 cut and paste options, nor search for them easily.
 [...]

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>