MastodonAPI.Controller.StatusController: fix dialyzer error

lib/pleroma/web/mastodon_api/controllers/status_controller.ex:333:pattern_match
The pattern can never match the type.

Pattern:
{:ok, _activity}

Type:
{:error, _}

lib/pleroma/web/common_api.ex

@@ -505,7 +505,7 @@ defp activity_is_public(activity) do
     end
   end
 
-  @spec unpin(String.t(), User.t()) :: {:ok, User.t()} | {:error, term()}
+  @spec unpin(String.t(), User.t()) :: {:ok, Activity.t()} | {:error, term()}
   def unpin(id, user) do
     with %Activity{} = activity <- create_activity_by_id(id),
          {:ok, unpin_data, _} <- Builder.unpin(user, activity.object),