wrong config map name in docs/examples/customization/custom-configuration/README.md

[ms4720]

Is this a request for help? (If yes, you should use our troubleshooting guide and community support channels, see https://kubernetes.io/docs/tasks/debug-application-cluster/troubleshooting/.):

What keywords did you search in NGINX Ingress controller issues before filing this one? (If you have found any duplicates, you should instead reply there.):

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Is this a BUG REPORT or FEATURE REQUEST? (choose one):
BUG REPORT

This is a minor doc bug

NGINX Ingress controller version:
N/A

Kubernetes version (use kubectl version):
N/A

Environment:

• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:

What happened:
readme gave bad config file name

What you expected to happen:
readme gives me the correct config file name

How to reproduce it (as minimally and precisely as possible):

N/A

Anything else we need to know:
Pull Opened on this ticket