[lib] Fix comment in kesyerver-types

Summary:
I noticed that the comment was incorrect. In the future, the cookie type will be one of string | undefined | null, but the field will have to be specified.
So the field declaration will need to be +cookie: ?string,.

Test Plan: N/A

Reviewers: michal, kamil

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8812