Fix ConfigMap's namespace in custom configuration example for nginx

[loicfrering]

The configmap option --configmap=$(POD_NAMESPACE)/nginx-custom-configuration in the nginx-custom-configuration.yaml deployment expects the ConfigMap to be in the kube-system namespace but it was not set in the ConfigMap definition.

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[k8s-reviewable]

This change is 

[loicfrering]

I just signed the CLA :)

[coveralls]

Coverage remained the same at 44.768% when pulling b469c19 on loicfrering:master into 1b41e80 on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@loicfrering thanks!

[loicfrering]

@aledbf You are very welcome, thanks a lot for your work 👍