Renaming Securing NGINX-ingress to ingress-nginx

[raghu-manne]

Securing NGINX-ingress can be renamed to Securing ingress-nginx

The mentioned tutorial uses ingress-nginx but the title says NGINX-ingress which can be confused with NGINX-ingress

[iacker]

Opened a docs PR to rename this tutorial (title, nav manifest entry, and in-body references) from "Securing NGINX-ingress" to "Securing ingress-nginx" so it matches the ingress-nginx project the tutorial actually uses and avoids confusion with the commercial NGINX Ingress Controller: #2188