customAuth is not a function

Hello. Forgive me for my bad english.

I'm trying to implement authentication and authorization in my graphql api. If I use :+1:
```
const customAuth = AuthDirective({
authenticateFunc: () => {
throw new AuthenticationError('Invalid token!');
}
});

const server = new ApolloServer({
typeDefs,
resolvers,
schemaDirectives: {
...customAuth
}
});
```

It's working. But I'm trying to use a custom function:

```
const server = new ApolloServer({
typeDefs,
resolvers,
schemaDirectives: {
auth: customAuth().isAuthenticated
}
});
```
The results is:
```
TypeError: customAuth is not a function
```

If I replace by
```
auth: customAuth.isAuthenticated
```

The directive is not working at all, but no error in console.

Any idea please ?