doc: edit fs.createReadStream() highWaterMark

Shorten the text about the `highWaterMark` value for the stream returned
by `fs.createReadStream()`.

PR-URL: https://github.com/nodejs/node/pull/20450


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>