Typo in README.md

Hi

There is a typo in `Example Playbook` section,
```
 roles:
     - michaelpporter.certbot-cloudflare
```

Should have underscore instead of hyphen between certbot and cloudflare naming
```
roles:
     - michaelpporter.certbot_cloudflare
```
*ansible-galaxy install michaelpporter.certbot_cloudflare*