Fix typo in apparmor_parser manpage

man apparmor_parser gives examples for the --warn command line option as

         apparmor_parser --warn=rules-not-enforced ...

and apparmor_parser --warn=no-rules-not-enforced ...

but the actual --warn options are rule-not-enforced / no-rule-not-enforced (without s)

Fixes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1057453

I propose this fix for 2.13..master