Fix collision of "i" at example in basics-server.wiki

[Yukimura66]

I found that following example in "Eliom: Typing page parameters" doesn't work.

let () =
  Eliom_registration.Html.register ~service:myservice
    (fun (s, i) () ->
      Lwt.return
         Eliom_content.Html.F.(html (head (title (txt "")) [])
                                    (body [h1 [txt (s^string_of_int i)]])))

In definition of html, symbol i is used for parameter, but this collide to element for list item. So I replaced symbol i to num, as following.

let () =
  Eliom_registration.Html.register ~service:myservice
    (fun (s, num) () ->
      Lwt.return
         Eliom_content.Html.F.(html (head (title (txt "")) [])
                                    (body [h1 [txt (s^string_of_int num)]])))