fix typo in TestMultiWriterSingleChainFlatten

Compare between `TestMultiReaderFlatten` and `TestMultiWriterSingleChainFlatten`, I guess that it should be `mw` instead of `w`.

src/io/multi_test.go

@@ -188,7 +188,7 @@ func TestMultiWriterSingleChainFlatten(t *testing.T) {
 	mw := w
 	// chain a bunch of multiWriters
 	for i := 0; i < 100; i++ {
-		mw = MultiWriter(w)
+		mw = MultiWriter(mw)
 	}
 
 	mw = MultiWriter(w, mw, w, mw)