docs: update old example (#1095)
docs: update old example (#1095)

Current usage according to this documentation of `isPostalCode` results in an error: 

```json
    "errors": {
        "message": "Invalid locale 'undefined'"
    }
```

According to #1085, a PR for v7 is on the way to improve this (?). Anyway, I'd appreciate it if you could update this in the documentation of v6 as well.
ctavan/express-validator,ctavan/express-validator