Fix hyphen-used-as-minus-sign in nyancat.1

Issue reported by Jakub Wilk using Debian Lintian tool.

http://lintian.debian.org/tags/hyphen-used-as-minus-sign.html

Signed-off-by: Jonathan McCrohan <jmccrohan@gmail.com>
Merge pull request #26 from jmccrohan/master

Fix hyphen-used-as-minus-sign in nyancat.1