Fix the link to rabbitmq.conf link to point to the file the does exist

[jasonholmberg]

This link to rabbitmq.conf at this page is broken:

https://www.rabbitmq.com/docs/oauth2-examples-okta#configure-rabbitmq-to-use-okta-as-oauth-20-authentication-backend

This PR attempts to fix that broken link by point to the rabbitmq.conf.tmpl file that does exist in that location.

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[michaelklishin]

For 4.1/main, it was addressed as a drive-by change in #2084.

[michaelklishin]

@jasonholmberg thank you!