[DOC] trivial fix for man page

I'm working on helping Arnaud update haproxy in Debian, and one of the
package build warnings I received was about "hyphen where a minus sign
was intended" in the man page - details:
http://lintian.debian.org/tags/hyphen-used-as-minus-sign.html

Patch included in my 1.3.20 Debian package is attached.

1 file changed