add missing permission in examples/iam-policy.json

[neerfri]

Adds a missing permission in the examples/iam-policy.json referenced from the walkthrough in the docs.

Fixes the error:

I0624 16:12:19.419873       1 targetgroup.go:316] [ALB-INGRESS] [default/some-ingress] [INFO]: Failed TargetGroup modification. Unable to change attributes: AccessDenied: User: arn:aws:sts::1122334455:assumed-role/my-role/session-name is not authorized to perform: elasticloadbalancing:ModifyTargetGroupAttributes on resource: arn:aws:elasticloadbalancing:us-east-1:1122334455:targetgroup/something-32555-HTTP-fd25f48/60bd46e9f1eeb72d
I0624 16:12:19.419885       1 targetgroup.go:316] [ALB-INGRESS] [default/some-ingress] [INFO]: 	status code: 403, request id: 5e9a919a-77c9-11e8-864b-c174c9d69364.
E0624 16:12:19.419893       1 albingress.go:332] [ALB-INGRESS] [default/some-ingress] [ERROR]: Failed to reconcile state on this ingress
E0624 16:12:19.419902       1 albingress.go:334] [ALB-INGRESS] [default/some-ingress] [ERROR]:  - AccessDenied: User: arn:aws:sts::1122334455:assumed-role/my-role/session-name is not authorized to perform: elasticloadbalancing:ModifyTargetGroupAttributes on resource: arn:aws:elasticloadbalancing:us-east-1:1122334455:targetgroup/something-32555-HTTP-fd25f48/60bd46e9f1eeb72d

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[rifelpet]

Based on this recent PR its going to need elasticloadbalancing:DescribeTargetGroupAttributes as well

[bigkraig]

@neerfri can you also include elasticloadbalancing:DescribeTargetGroupAttributes and keep it sorted alphabetically?