Fix ExtensionCodec example so it recurses.

[jcc10]

Fixes example so data doesn't get lost in recursion.

Creates valid workaround for #138

Spent about a hour before I realized the example was broken. 🤦‍♀️

[gfx]

Thank you! Yes, you are right, and I have also experienced the same issue before. It is a wrong design of the current interface, which I'd love to fix in the future.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (e615511) 98.58% compared to head (29a970f) 98.58%.

:exclamation: Current head 29a970f differs from pull request most recent head 07900e5. Consider uploading reports for the commit 07900e5 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #225 +/- ## ======================================= Coverage 98.58% 98.58% ======================================= Files 16 16 Lines 990 990 Branches 195 195 ======================================= Hits 976 976 Misses 14 14 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=msgpack). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=msgpack)

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