docs(ChainType): fix docs for chaintype return value

[TarikGul]

closes: [#624]

I updated the docs to reflect the actual return value of ChainType via polkadot-js.

The route this is associated with is /runtime/spec.

The enum ChainType has 4 variants:

- Live => null
- Development => null
- Local => null
- Custom => string

The old docs reflected that the return value is a string, but instead an object is returned with the key as one of the variants, and the value as null except when the variant custom is used, in which case a string will be the value.