fix README.md with updated 'Allowed Callback URLs'

[ynovytskyy]

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Description

Following README as is does not work. Apparently the okta library has been updated in the sample app and is using a different callback URL ending in /okta instead of /auth0 as instructed to configure the app here.

References

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Testing

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Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not master