bugfix: remove 'name' in snippets' required field and add 'documentNamespace' to doc's required field

[ammend]

remove 'name' in snippets' required field and add 'documentNamespace' to doc's required field according to official spdx website. By the link below, 'documentNamespace' is required in the specification link however it not not required in json schema. And 'name' is not required in the specification link however it is required in json schema.

[bact]

The PR failed because there's no signed-off information in the commit message.

To fix it, see: https://github.com/spdx/spdx-spec/pull/1021/checks?check_run_id=28332639903

[ammend]

remove 'name' in snippets' required field and add 'documentNamespace' to doc's required field according to official spdx website. By the link below, 'documentNamespace' is required in the specification link however it not not required in json schema. And 'name' is not required in the specification link however it is required in json schema.

@goneall Thanks for your review