Use \- as minus in manpage

From [0]

    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.

So "\-" shall be used when minus is needed.

[0] http://lintian.debian.org/tags/hyphen-used-as-minus-sign.html
Merge pull request #83 from czchen/bugfix/manpage

Use \- as minus in manpage