doc: fix process.stdin example
Fixes: https://github.com/nodejs/node/issues/20503
Checklist
-
make -j4 test(UNIX), orvcbuild test(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Fixes: https://github.com/nodejs/node/issues/20503
make -j4 test (UNIX), or vcbuild test (Windows) passes