docs: fix stream async iterator sample
The for await loop into writable could cause an unhandled exception
in the case where we are waiting for data from the async iterable and
there is no 'error'
handler registered on the writable.
Fixes: https://github.com/nodejs/node/issues/31222
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines