doc: fix unit of size argument of readable.read

"GB" refers to 10**9 bytes, whereas the actual limit is 2**30. The
correct unit symbol is "GiB".

PR-URL: https://github.com/nodejs/node/pull/35051


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>