doc: fix example in stream doc

Fix cases where `chunk` is a buffer.

PR-URL: https://github.com/nodejs/node/pull/8378
Fixes: https://github.com/nodejs/node/issues/8085
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/stream.md

@@ -1643,8 +1643,8 @@ class MyDuplex extends Duplex {
   _write(chunk, encoding, callback) {
     // The underlying source only deals with strings
     if (Buffer.isBuffer(chunk))
-      chunk = chunk.toString(encoding);
-    this[kSource].writeSomeData(chunk, encoding);
+      chunk = chunk.toString();
+    this[kSource].writeSomeData(chunk);
     callback();
   }