Fix a broken url

[ooooo-create]

ref #2576

The docs/extensions.md have changed to docs/extensions/readme.md.

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@ooooo-create : Thanks for your contribution! The author(s) have been notified to review your proposed change.

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Learn Build status updates of commit d608c9f:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
hub/dev-home/extensions.md	:white_check_mark:Succeeded

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[mattwojo]

Thanks for the fix @ooooo-create !