Fix JSON seeAlsos property

[nicoweidner]

• Should be called seeAlsos in the written JSON instead of seeAlso, see https://github.com/spdx/spdx-spec/blob/v2.3/schemas/spdx-schema.json#L172
• Multiple entries are currently concatenated into a single string, which is wrong. Maybe only when converting from tag/value to json

[nicoweidner]

While analyzing, I realized the second issue is really an issue with the example from spdx-spec, see spdx/spdx-spec#823