Fix `panic: alert_grouping_timeout: '' expected type 'string', got un…

…convertible type 'int'`:

```
panic: alert_grouping_timeout: '' expected type 'string', got unconvertible type 'int'

goroutine 288 [running]:
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*ResourceData).Set(0xc000188900, {0x1065e32, 0x16}, {0xec9ec0, 0xc0006aa8e8})
        /home/patrick/go/src/github.com/PagerDuty/terraform-provider-pagerduty/vendor/github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema/resource_data.go:230 +0x2a5
github.com/terraform-providers/terraform-provider-pagerduty/pagerduty.resourcePagerDutyServiceRead.func1()
        /home/patrick/go/src/github.com/PagerDuty/terraform-provider-pagerduty/pagerduty/resource_pagerduty_service.go:396 +0x508
github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource.RetryContext.func1()
        /home/patrick/go/src/github.com/PagerDuty/terraform-provider-pagerduty/vendor/github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource/wait.go:27 +0x56
github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource.(*StateChangeConf).WaitForStateContext.func1()
        /home/patrick/go/src/github.com/PagerDuty/terraform-provider-pagerduty/vendor/github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource/state.go:110 +0x21f
created by github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource.(*StateChangeConf).WaitForStateContext
        /home/patrick/go/src/github.com/PagerDuty/terraform-provider-pagerduty/vendor/github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource/state.go:83 +0x1dd
FAIL    github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   5.947s
FAIL
make: *** [GNUmakefile:17: testacc] Error 1
```

pagerduty/resource_pagerduty_service.go

@@ -393,7 +393,7 @@ func resourcePagerDutyServiceRead(d *schema.ResourceData, meta interface{}) erro
 		if service.AlertGroupingTimeout == nil {
 			d.Set("alert_grouping_timeout", "null")
 		} else {
-			d.Set("alert_grouping_timeout", *service.AlertGroupingTimeout)
+			d.Set("alert_grouping_timeout", strconv.Itoa(*service.AlertGroupingTimeout))
 		}
 		if service.AlertGroupingParameters != nil {
 			if err := d.Set("alert_grouping_parameters", flattenAlertGroupingParameters(service.AlertGroupingParameters)); err != nil {