doc: fix missing negation in stream.md

Changing `..can result in expected` to `..can result in unexpected`

Fixes: https://github.com/nodejs/node/issues/10710
PR-URL: https://github.com/nodejs/node/pull/10712


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>