fix(curriculum): Backend challenge: Create user URL in exercise tracker must be without slash at the end

[alexandear]

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This PR fixes the URL to create a user in the exercise tracker backend challenge. The URL is without a slash at the end url + /api/users, not url + /api/users/.

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[raisedadead]

You do not need to keep updating the branch. We will ask you for a rebase if needed. Someone will review this soon.

If you have an issue/discussion about this PR that you can link to, you should add it to the description; else it is okay.

Thanks.