d/kubernetes_service: fix broken schema

[appilon]

switch target_port to string

This broke from another PR
BEFORE:

=== RUN   TestAccKubernetesDataSourceService_basic
--- FAIL: TestAccKubernetesDataSourceService_basic (0.89s)
	testing.go:518: Step 0 error: Error applying: 1 error(s) occurred:

		* data.kubernetes_service.test: data.kubernetes_service.test: spec.0.port.0.target_port: '' expected type 'int', got unconvertible type 'string'
	testing.go:579: Error destroying resource! WARNING: Dangling resources
		may exist. The full state and error is shown below.

		Error: Error refreshing: 1 error(s) occurred:

		* data.kubernetes_service.test: 1 error(s) occurred:

		* data.kubernetes_service.test: data.kubernetes_service.test: spec.0.port.0.target_port: '' expected type 'int', got unconvertible type 'string'

		State: <nil>

AFTER

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./kubernetes -v -run=TestAccKubernetesDataSourceService_basic -timeout 120m
=== RUN   TestAccKubernetesDataSourceService_basic
--- PASS: TestAccKubernetesDataSourceService_basic (1.26s)
PASS
ok  	github.com/terraform-providers/terraform-provider-kubernetes/kubernetes	1.355s

[radeksimko]

It would be nice to add an acceptance test which fails without this patch.

[appilon]

@radeksimko the test I show passing was actually broken I just did not realize it until I ran the full suite of tests. I've updated the description with BEFORE and AFTER