docs: further fix async iterator example
Further fixes an issue with the async iterator example where an incorrect assumption was made in regards that drain or error is always invoked after !write().
Fixes: https://github.com/nodejs/node/issues/31365
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