xz man page: Change \- (minus) to \(en (en-dash) for a numeric range.

Docs of ancient troff/nroff mention \(em (em-dash) but not \(en
and \- was used for both minus and en-dash. I don't know how
portable \(en is nowadays but it can be changed back if someone
complains. At least GNU groff and OpenBSD's mandoc support it.

Thanks to Bjarni Ingi Gislason for the patch.