Update EnrollAccount README.md with examples

I'd like to update the EnrollAccount [README.md](https://github.com/aws-samples/aws-control-tower-reference-architectures/blob/master/customizations/AccountFactory/EnrollAccount/README.md) with an **Examples** section:

### Examples
When using the `-n` option, you'll still use the `-o` and `-u` options, you'll just need to use the OU id. Example:
```
enroll_account.py -o "ou-ab12-cd345ef" -u "ou-cd56-7890zyxw" -n
```

Open customizations/AccountFactory/EnrollAccount/README.md and review the existing usage guidance for enroll_account.py. Add an Examples section showing -n together with -o and -u using OU IDs, including the command from the issue. Done means the README clearly documents this invocation.

Written by the indexing model from the issue text.