Healthcheck port is optional

Correct comment in `load_balancer_http-input.yaml` to state that the
healthcheck port is optional as per:
https://github.com/alphagov/vcloud-edge_gateway/blob/e0c3685fc51b8b52de4871c7d314b86fe640c212/lib/vcloud/edge_gateway/schema/load_balancer_service.rb#L10