[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

lib/types/keyserver-types.js

@@ -5,7 +5,7 @@ import t, { type TInterface } from 'tcomb';
 import { tShape } from '../utils/validation-utils.js';
 
 // Once we start using the cookie field on web,
-// the cookie field type should be changed to string | null.
+// the cookie field should be mandatory, of type ?string.
 // See https://linear.app/comm/issue/ENG-4347/stop-using-browser-cookies
 export type KeyserverInfo = {
   +cookie?: ?string,