doc: fix keyObject.symmetricSize to be keyObject.symmetricKeySize

PR-URL: https://github.com/nodejs/node/pull/25670


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>