Fix "NotImplentedError" typo in constants documentation

[zertrin]

NotImplentedError --> NotImplementedError

[mention-bot]

@zertrin, thanks for your PR! By analyzing the history of the files in this pull request, we identified @birkenfeld, @tiran and @ethanfurman to be potential reviewers.

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[zertrin]

Hi @the-knights-who-say-ni
The CLA has been signed and validated ;)