docs: ingress namespace is different with apisix and apisix-dashboard

[zaunist]

In our document, apisix and apisix-dashboard is installed to default namespace, but the apisix-ingress-controller is installed to ingress-apisix namespace. If we just install these charts according to official document, apisix-ingress-controller cannot be started normally because it is not in the same namespace as apisix.

[tao12345666333]

you should add --set ingress-controller.config.apisix.serviceNamespace=<your namespace> flag.

[tao12345666333]

see https://github.com/apache/apisix-helm-chart/issues/176#issuecomment-960422141

welcome to improve the documentation.

[xewn]

#236 我觉得这已经是bug了，搞成一样的，就会少一些麻烦。很多用户都是头次接触，根本不懂K8S的使用。