doc: correct getting an ArrayBuffer's length

`ArrayBuffer` instances do not have `.length` property.
Instead they have `.byteLength` property.
Fixed that in the description of
`new Buffer(arrayBuffer[, byteOffset[, length]])` and
`Buffer.from(arrayBuffer[, byteOffset[, length]])`.

PR-URL: https://github.com/nodejs/node/pull/31632


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>