Simplify type-choice-items ABNF

```
type-choice-items = *sp-cmt ( type-choice / type-rule ) *sp-cmt
```
can be simplified to:
```
type-choice-items = *sp-cmt type-rule *sp-cmt
```
because `type-rule` has `type-choice` in it already:
```
type-rule        = value-rule / type-choice / target-rule-name
```