doc: fix the example implementation of MemoryRetainer

We need to be careful not to include the size of non-pointer
fields in the parent's self size if we want to track them separately
as a different node.

Refs: https://github.com/nodejs/node/pull/26161/files#r259170771

PR-URL: https://github.com/nodejs/node/pull/26262


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>