doc: fix unsafe writable stream code examples
Update writable stream code examples using async iterator to use safer
finished()
method instead of the finish
event to avoid uncaught
errors
Fixes: https://github.com/nodejs/node/issues/29397
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines