Fix snapshot.create response

[pquentin]

I'm not sure when this was added, but we only notice it now because of the new YAML test added in elastic/elasticsearch#113233.

This field is also used in snapshot.get.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
snapshot.cleanup_repository	🟢	3/3	3/3
snapshot.clone	🟢	5/5	5/5
snapshot.create_repository	🔴	24/29	29/29
snapshot.create	🟢	28/28	28/28
snapshot.delete_repository	🟢	10/10	10/10
snapshot.delete	🔴	21/23	23/23
snapshot.get_repository	🔴	19/19	7/19
snapshot.get	🟢	13/13	13/13
snapshot.repository_analyze	🟠	Missing type	Missing type
snapshot.restore	🟢	5/5	5/5
snapshot.status	🟢	2/2	2/2
snapshot.verify_repository	🟢	2/2	2/2

You can validate these APIs yourself by using the make validate target.