Code review comment for ~jslarraz/review-tools:fix-custom-device-info

Thanks for this @jslarraz - but I still see the read-devices slot attribute referenced in upstream snapd - https://github.com/snapcore/snapd/blob/master/interfaces/builtin/custom_device.go#L246 - so I think this should be retained - but the change from content to custom-device for the plug attribute looks right.