doc: fix incorrect name in report docs

In diagnostic reports, the CPUs are listed in a "cpus" field.
This commit fixes the docs, which refer to the field as "osCpus"

PR-URL: https://github.com/nodejs/node/pull/28830


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>