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

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)
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Merged revisions 423915 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 423916 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 423917 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 423918 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423919 65c4cc65-6c06-0410-ace0-fbb531ad65f3