Renaming Securing NGINX-ingress to ingress-nginx

[Securing NGINX-ingress](https://cert-manager.io/docs/tutorials/acme/nginx-ingress/) can be renamed to **Securing ingress-nginx**


The mentioned tutorial uses [ingress-nginx](https://kubernetes.github.io/ingress-nginx) but the title says **NGINX-ingress** which can be confused with  [NGINX-ingress](https://docs.nginx.com/nginx-ingress-controller/overview/about/)