Update README.md

[rakvium]

with correct name for SolidusStripe::Webhook::Event::CORE_EVENTS constant

Summary

SolidusStripe::Webhook::Event::CORE returns uninitialized constant SolidusStripe::Webhook::Event::CORE (NameError)

Checklist

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• [x] I agree that my PR will be published under the same license as Solidus.
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• 📖 I have updated the README to account for my changes.
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