Fix merge request diff cache service spec

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

spec/services/merge_requests/merge_request_diff_cache_service_spec.rb

@@ -6,7 +6,7 @@ describe MergeRequests::MergeRequestDiffCacheService do
   describe '#execute' do
     it 'retrieves the diff files to cache the highlighted result' do
       merge_request = create(:merge_request)
-      cache_key = [merge_request.merge_request_diff, 'highlighted-diff-files', Gitlab::Diff::FileCollection::MergeRequest.default_options]
+      cache_key = [merge_request.merge_request_diff, 'highlighted-diff-files', Gitlab::Diff::FileCollection::MergeRequestDiff.default_options]
 
       expect(Rails.cache).to receive(:read).with(cache_key).and_return({})
       expect(Rails.cache).to receive(:write).with(cache_key, anything)