Fix spec error class

Author: pitr-ch

spec/concurrent/edge/promises_spec.rb

@@ -221,7 +221,7 @@ def behaves_as_delay(delay, value)
       let(:a_future) { future { raise 'error' } }
 
       it 'raises a concurrent error' do
-        expect { zip(a_future).value! }.to raise_error(Concurrent::Error)
+        expect { zip(a_future).value! }.to raise_error(StandardError)
       end
 
     end