Clarify validation rule on non-null type wrapping another non-null type

The [specification](https://spec.graphql.org/October2021/#sel-HAHlBLDBABAB0B7gY) states that:
> A Non-Null type must not wrap another Non-Null type.

In the specification we see the type `[Int!]!` used in [an example](https://spec.graphql.org/October2021/#sel-KAHlBNJDZBAABABHu0Z), which is a non-null list type that wraps another non-null named type.

The validation rule cannot hold and the example be valid simultaneously.