Adds documentation about GHE_EXTRA_SSH_OPTS in example config file

I need to specify a particular SSH key to communicate with my GitHub enterprise server:

```ssh -i /path/to/non/default/key admin@ghe_hostname.example.com```

I dug into the code and saw that I could easily specify this by setting:
```GHE_EXTRA_SSH_OPTS="-i /path/to/non/default/key"``` in backup config.

It would have been useful to me if this option was documented as being available in the example configuration file.