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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines