fix: add missing case in spec of multi_search

[davidjulien]

Meilisearch.MultiSearch.multi_search calls Meilisearch.Client.handle_response() (here) which can return {:error, Meilisearch.Error.cast(body), status} (here)

Because this case is missing in multi_search spec (here), dialyxir returns this error when multi_search fun is used:

The pattern can never match the type.

Pattern:
{:error, _error, _status}

Type:

  {:error,
   Tesla.Error
   | nil
   | %Meilisearch.Error{
       :code => atom(),
       :link => binary(),
       :message => binary(),
       :type => :auth | :internal | :invalid_request | :system
     }}