Fix directive names for auth_http_*

[puneetsarna]

https://nginx.org/en/docs/mail/ngx_mail_auth_http_module.html

Proposed changes

These directives are not valid nginx directives and the change fixes these directives
https://nginx.org/en/docs/mail/ngx_mail_auth_http_module.html

Checklist

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• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md)