fix prefixLength regex pattern in prefixclaim

[faebr]

The following was a prefixClaim accepted by kube-api. The regex needs brackets around the or statement so the startline and endline characters in the regex always apply.

---
apiVersion: netbox.dev/v1
kind: PrefixClaim
metadata:
  labels:
    app.kubernetes.io/name: netbox-operator
    app.kubernetes.io/managed-by: kustomize
  name: prefixclaim-sample
spec:
  tenant: "Dunder-Mifflin, Inc."
  site: "DM-Akron"
  description: "some description"
  comments: "your comments"
  preserveInNetbox: true
  parentPrefix: "2.0.0.0/16"
  prefixLength: "/222222222222222222222"