Fix RSpec/MissingExampleGroupArgument cop (#25310)

spec/lib/activitypub/activity/undo_spec.rb

@@ -31,7 +31,7 @@ RSpec.describe ActivityPub::Activity::Undo do
         }
       end
 
-      context do
+      context 'when not atomUri' do
         before do
           Fabricate(:status, reblog: status, account: sender, uri: 'bar')
         end