doc: fix the spelling error in stream.md

Change `64kb` to `64KB` in  `stream.md`

PR-URL: https://github.com/nodejs/node/pull/31561


Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>