prctl.2: ffix: use literal hyphens when referencing kernel docs

prctl.2: ffix: use literal hyphens when referencing kernel docs

There is one case of a cross-reference to a kernel documentation
filename that uses unescaped hyphens.

To avoid misrendering, escape these as \- similarly to other
instances.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>