doc: update and modernize examples in fs.ms

* unify quotes in fs.md
* avoid quote escaping in fs.md
* simplify logics in fs.md
* concatenation -> template literal in fs.md
* add missing callback in fs.md
* fix typo in fs.md
* update output example in fs.md

PR-URL: https://github.com/nodejs/node/pull/12035


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>