Update saml-authentication.asciidoc

[kurtiam]

updated api/security/v1/saml to /api/security/saml/callback in example code to reflect non-deprecated setting for sp.acs:

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?
• If you submit code, is your pull request against main? We recommend pull requests against main. We will backport them as needed.

[naemono]

This is in-line with the current Elasticsearch Documentation

Thanks for this @kurtiam

[naemono]

@elasticmachine run elasticsearch-ci/docs

[naemono]

This is the only place I see this example, so this appears to be 👍