Example code for setting cookies

Nobody has claimed this yet.

The [documentation for cookies](https://github.com/luminus-framework/luminus/blob/master/resources/md/sessions_cookies.md#cookies) shows that cookies can be set like this:

``` clj
(-> "cookie set" response (update-in [:cookies "username" :value] "Alice"))
```

I don't think `(response "cookie set")` makes sense and `"Alice"` cannot be used as a function for `update-in`. I think the example can be simply this:

``` clj
(assoc-in response [:cookies "username" :value] "Alice")
```

Do I understand it correctly?

Open resources/md/sessions_cookies.md at the cookies section and review the surrounding examples and terminology. Verify the proposed cookie-update expression against the documented response structure, then update the example so it is valid and consistent with the guide. Done means the snippet can be followed without calling a string as a function.

Written by the indexing model from the issue text.