pkg/failpoint: add FreeBSD link and update pkg doc
pkg/failpoint: add FreeBSD link and update pkg doc

Signed-off-by: Wei Fu <df4176d36abdc6fd0d966de5d5471522668de494@gmail.com>

estesp/containerd,thaJeztah/containerd,mikebrow/containerd,estesp/containerd,containerd/containerd,thaJeztah/containerd,mikebrow/containerd,containerd/containerd