Fix: Wrong reference for Docker Certmanager - 404

[iagonc]

this reference to docker certmanager is incorrect as the repository apparently no longer exists:

docs-gluu-server-prod/docs/source/reference/container-image-refs.md

Line 25 in 32f611e

[Gluu Cert Manager](https://github.com/GluuFederation/docker-certmanager/blob/4.4/README.md)

[nynymike]

Thx. BTW, iFoods can also raise a support issue.

[iagonc]

Thx. BTW, iFoods can also raise a support issue.

Thanks for the reply, I sent a PR fixing the references :)