conceptScheme parent is ID reference, but text says URN

[hoehrmann]

In https://github.com/sdmx-twg/sdmx-json/blob/71fe5eaa9fcd29e3c15f2f0216a19b9b650b1dbd/structure-message/docs/1-sdmx-json-field-guide.md#conceptscheme defines »parent - String optional. Urn reference to a local concept.« but lexically it is an ID reference according to the schema and the example in this section. The example that follows also seems broken to me, COMMON_CONCEPTS is probably not a good ID for a concept, and the example does not actually include this concept.

(In doubt, please handle this as a public review comment on SDMX 3.1 once the comment period begins.)