NamespaceMap: fix TODO; "ElementID" -> "Element ID"

[bact]

• Complete TODO on serialization information and NamespaceMap
  • Now link to https://spdx.github.io/spdx-spec/v3.0.1-draft/serializations/ (the path is relative, so it will be "v3.0.1" once v3.0.1 is published)
• Use 4-space indent in the numbered list, to keep the intended multiple-paragraph format
• "ElementID" -> "Element ID"
  • It is not a specific name.
  • "elementId" was found used as a property of an ExternalMap in code examples of these two issues from late 2022/early 2023:
    • Can one parse a JSON containing Elements extended by an unsupported / unknown profile? #42
    • How to add profile data to an existing SBOM or Element #61
    • But that property is more likely being called externalSpdxId now.
  • Another instance of "ElementID" is found in tools-golang
  • Cannot find "ElementID" anywhere else in the spec and the model. Only found in NamespaceMap and namespaceMap.

This is a rework of #796

[goneall]

Changes look good - I did notice that we have a link to the SPDX 3 model repo without a specific version or commit. We can open a separate PR or issue if this needs to be solved for the ISO submission.