fix: mistaken auth.js integration callback url (remove /api prefix)

[wassimoo]

Related Issue or Design Document

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
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• I have added tests that prove my fix is effective or that my feature works.
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Further comments

[wassimoo]

Documented some of the issues I faced through the guide. Also, updated the corresponding sample repo, check ory/next-auth-example#4

[vinckr]

lgtm
didn't test the auth.js example e2e