test: update comments in test-fs-read-offset-null

Update comment to refer to the correct ASCII code (120 rather than 66).
All other changes are cosmetic.

PR-URL: https://github.com/nodejs/node/pull/36152


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>