Fix copy/paste typo in CodedInputStreamTest
diff --git a/csharp/src/Google.Protobuf.Test/CodedInputStreamTest.cs b/csharp/src/Google.Protobuf.Test/CodedInputStreamTest.cs
index 239d3c9..c0a9ffd 100644
--- a/csharp/src/Google.Protobuf.Test/CodedInputStreamTest.cs
+++ b/csharp/src/Google.Protobuf.Test/CodedInputStreamTest.cs
@@ -589,7 +589,7 @@
         {

             var memoryStream = new MemoryStream();

             Assert.IsTrue(memoryStream.CanRead);

-            using (var cos = new CodedOutputStream(memoryStream, true))

+            using (var cis = new CodedInputStream(memoryStream, true))

             {

             }

             Assert.IsTrue(memoryStream.CanRead); // We left the stream open