Fix OpenAPI spec for preferred_frontend endpoint

The spec was copied from another endpoint, including the operation id,
leading to scrubbing the valid parameters from the request and simply
not working.

Signed-off-by: nicole mikołajczyk <git@mkljczk.pl>

test/pleroma/web/pleroma_api/controllers/frontend_settings_controller_test.exs

@@ -0,0 +1,19 @@
+defmodule Pleroma.Web.PleromaAPI.FrontendSettingsControllerTest do
+  use Pleroma.Web.ConnCase, async: false
+
+  import Pleroma.Factory
+
+  describe "PUT /api/v1/pleroma/preferred_frontend" do
+    test "sets a cookie with selected frontend" do
+      %{conn: conn} = oauth_access(["read"])
+
+      response =
+        conn
+        |> put_req_header("content-type", "application/json")
+        |> put("/api/v1/pleroma/preferred_frontend", %{"frontend_name" => "pleroma-fe/stable"})
+
+      json_response_and_validate_schema(response, 200)
+      assert %{"preferred_frontend" => %{value: "pleroma-fe/stable"}} = response.resp_cookies
+    end
+  end
+end