doc: `readFileSync` instead of `fs.readFileSync`

The ALPN negotiation sample uses `fs.readFileSync` whereas
`readFileSync` is imported into top level variable.

PR-URL: https://github.com/nodejs/node/pull/15137


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>