Native API: Dataset Migration endpoint has outdated reference to a bug.

[kcondon]

The dataset migration endpoint doc has an outdated reference to a now fixed bug: (Also note that Issue #8028 currently breaks testing this API with DataCite test DOIs.)
https://guides.dataverse.org/en/5.9/developers/dataset-migration-api.html#start-migrating-a-dataset-into-a-dataverse-collection
It should be removed.