stream: fix finished typo

https://github.com/nodejs/node/pull/31509 introduced a slight typo.
Fortunately this typo does not have big impact due to
`isWritableFinished()`.

Fixes: https://github.com/nodejs/node/pull/31509#discussion_r381809355

PR-URL: https://github.com/nodejs/node/pull/31881
Fixes: https://github.com/nodejs/node/issues/31509


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>