Fix RSpec/VerifiedDoubles cop (#25469)

spec/services/bootstrap_timeline_service_spec.rb

@@ -6,7 +6,7 @@ RSpec.describe BootstrapTimelineService, type: :service do
   subject { described_class.new }
 
   context 'when the new user has registered from an invite' do
-    let(:service)    { double }
+    let(:service)    { instance_double(FollowService) }
     let(:autofollow) { false }
     let(:inviter)    { Fabricate(:user, confirmed_at: 2.days.ago) }
     let(:invite)     { Fabricate(:invite, user: inviter, max_uses: nil, expires_at: 1.hour.from_now, autofollow: autofollow) }