doc: use backticks around file names in README.md

Use backticks around `SHASUM256.txt` etc. in README.md.

PR-URL: https://github.com/nodejs/node/pull/23299


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>