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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines