doc: typo in stream.md

* **Version**: master
* **Platform**: n/a
* **Subsystem**: doc

There is a typo in doc/api/stream.md in the [`stream._transform()`](https://github.com/nodejs/node/blob/0e5283b7eda5b22d73c21620a02ac5337f9de9f1/doc/api/stream.md#transform_transformchunk-encoding-callback) description: 'asychronously' should be 'asynchronously'.
