Fix `pagerduty_service.foo: Attribute 'alert_grouping_timeout' found …

…when not expected`:

```
=== RUN   TestAccPagerDutyService_Basic
    resource_pagerduty_service_test.go:57: Step 1/3 error: Check failed: Check 8/12 error: pagerduty_service.foo: Attribute 'alert_grouping_timeout' found when not expected
--- FAIL: TestAccPagerDutyService_Basic (11.33s)
```

pagerduty/resource_pagerduty_service_test.go

@@ -75,8 +75,8 @@ func TestAccPagerDutyService_Basic(t *testing.T) {
 						"pagerduty_service.foo", "alert_creation", "create_incidents"),
 					resource.TestCheckNoResourceAttr(
 						"pagerduty_service.foo", "alert_grouping"),
-					resource.TestCheckNoResourceAttr(
-						"pagerduty_service.foo", "alert_grouping_timeout"),
+					resource.TestCheckResourceAttr(
+						"pagerduty_service.foo", "alert_grouping_timeout", "null"),
 					resource.TestCheckResourceAttr(
 						"pagerduty_service.foo", "incident_urgency_rule.#", "1"),
 					resource.TestCheckResourceAttr(