Escape minus signs in manpage

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.

(Reported by lintian.)

Signed-off-by: Loc Minier <loic.minier@linaro.org>