json-format-specification is missing group type of gedcomx type

[eneufeld]

The https://github.com/FamilySearch/gedcomx/blob/master/specifications/json-format-specification.md#43-the-gedcomx-data-type is missing the Group Data Type which is defined in the conceptional model: https://github.com/FamilySearch/gedcomx/blob/master/specifications/conceptual-model-specification.md#group

[stoicflame]

Thanks. Fixed at 826d723.