Shallow merge "externalDocumentId" is not implemented according to spdx-spec/v2.3

[vchandap]

Hi
we are trying shallow merge with SPDXMerge and found that externalDocumentId should have "DocumentRef-" and "unique string"

Instead, it has only SPDX-ID

Reference

https://spdx.github.io/spdx-spec/v2.3/document-creation-information/#66-external-document-references-field

Thank you.