Fix RSpec/MultipleSubjects cop (#24738)

spec/controllers/activitypub/outboxes_controller_spec.rb

@@ -35,10 +35,9 @@ RSpec.describe ActivityPub::OutboxesController, type: :controller do
 
   describe 'GET #show' do
     context 'without signature' do
-      subject(:body) { body_as_json }
-
       subject(:response) { get :show, params: { account_username: account.username, page: page } }
 
+      let(:body) { body_as_json }
       let(:remote_account) { nil }
 
       context 'with page not requested' do