Fix certificate revocation in README.md

The README.md file has been updated to clarify the process of revoking certificates. The previous instructions were incorrect, and the correct method is now provided. The certificates can be revoked using the `certs` service by providing the `thing_id` of the thing for which the certificate was issued.

```
curl -s -S -X DELETE http://localhost:9019/certs/revoke -H "Authorization: Bearer $TOK" -H 'Content-Type: application/json' -d '{"thing_id":"c30b8842-507c-4bcd-973c-74008cef3be5"}'
```

Signed-off-by: Rodney Osodo <socials@rodneyosodo.com>

certs/README.md

@@ -20,8 +20,7 @@ MF_CERTS_VAULT_TOKEN=<vault_acces_token>
 
 For lab purposes you can use docker-compose and script for setting up PKI in [https://github.com/mteodor/vault](https://github.com/mteodor/vault)
 
-Issuing certificate is same as in **Development** mode.
-In this mode certificates can also be revoked:
+The certificates can also be revoked using `certs` service. To revoke a certificate you need to provide `thing_id` of the thing for which the certificate was issued.
 
 ```bash
 curl -s -S -X DELETE http://localhost:9019/certs/revoke -H "Authorization: Bearer $TOK" -H 'Content-Type: application/json'   -d '{"thing_id":"c30b8842-507c-4bcd-973c-74008cef3be5"}'