docs: Escape unescaped minus sign in asterisk.8 manpage.

ASTERISK-23768 #close
Reported by: Jeremy Lainé
Patches:
  escape_manpage_hyphen.patch uploaded by Jeremy Lainé (License #6561)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@423915 f38db490-d61c-443f-a65b-d21fe96a405b