Updated authentication docs to reflect breaking changes

[prespic]

Authentication metadata decorator was changed to be able set more than one authentication strategy. https://github.com/loopbackio/loopback-next/commit/ae6c0e68a58a2b574fd534242e599aa2a96fc855

However, the example in docs was not updated and currenty it is not working.

See also #5735

Checklist

• [ ] DCO (Developer Certificate of Origin) signed in all commits
• [ ] npm test passes on your machine
• [ ] New tests added or existing tests modified to cover all changes
• [ ] Code conforms with the style guide
• [ ] API Documentation in code was updated
• [ ] Documentation in /docs/site was updated
• [ ] Affected artifact templates in packages/cli were updated
• [ ] Affected example projects in examples/* were updated

👉 Check out how to submit a PR 👈

[dhmlau]

@prespic, thanks for the PR. The change LGTM. Could you please sign the DCO so that I can help landing your PR? Thanks.