Update staging.md to use ACMEv2 server (closes #812)

Update staging.md to use ACMEv2 server (closes #812)

letsencrypt is phasing out the v1 server:

```
  + ERROR: An error occurred while sending get-request to https://acme-staging.api.letsencrypt.org/directory (Status 403)

Details:
HTTP/2 403
server: nginx
date: Thu, 01 Apr 2021 20:48:17 GMT
content-type: application/problem+json
content-length: 189
etag: "600b3710-bd"

{
  "type": "urn:acme:error:serverInternal",
  "detail": "ACMEv1 Brownout in Progress. ACMEv1 will fully turn off on June 1, 2021. Check https://letsencrypt.status.io/ for more details."
}

```