This is a Bug Report

Problem:
At the very beginning of the referred doc. link it is said:
"Operators are software extensions to Kubernetes that make use of third-party resources to manage applications and their components"
But third-party resources (TPR) are now named custom resources (from 1.7 -as far as I remember-)

Proposed Solution:
"Operators are software extensions to Kubernetes that make use of custom resources to manage applications and their components"

Page to Update:
https://kubernetes.io/docs/concepts/extend-kubernetes/operator/